diff --git a/src/factory.ts b/src/factory.ts
new file mode 100644
index 0000000..42be3f9
--- /dev/null
+++ b/src/factory.ts
@@ -0,0 +1,49 @@
+import { ResolvedFields, DefaultFieldsResolver, InputFieldsResolver, resolveFields } from './field-resolver.js';
+import { getSequenceCounter, resetSequence } from './sequence.js';
+import { Merge } from './util.js';
+
+export interface TypeFactoryDefineOptions<
+  Type extends Record<string, unknown>,
+  TransientFields extends Record<string, unknown>,
+> {
+  defaultFields: DefaultFieldsResolver<Type & TransientFields>;
+}
+export interface TypeFactoryInterface<
+  Type extends Record<string, unknown>,
+  TransientFields extends Record<string, unknown>,
+  Options extends TypeFactoryDefineOptions<Type, TransientFields>,
+> {
+  build(): Promise<Pick<Merge<ResolvedFields<Options['defaultFields']>, ResolvedFields<{}>>, keyof Type>>;
+  build<const T extends InputFieldsResolver<Type & TransientFields>>(
+    inputFieldsResolver: T,
+  ): Promise<Pick<Merge<ResolvedFields<Options['defaultFields']>, ResolvedFields<T>>, keyof Type>>;
+  resetSequence(): void;
+}
+
+export function defineTypeFactoryInternal<
+  Type extends Record<string, unknown>,
+  TransientFields extends Record<string, unknown>,
+  Options extends TypeFactoryDefineOptions<Type, TransientFields>,
+>(
+  typeFieldNames: readonly (keyof Type)[],
+  { defaultFields: defaultFieldsResolver }: Options,
+): TypeFactoryInterface<Type, TransientFields, Options> {
+  const seqKey = {};
+  const getSeq = () => getSequenceCounter(seqKey);
+  return {
+    async build<const T extends InputFieldsResolver<Type & TransientFields>>(
+      inputFieldsResolver?: T,
+    ): Promise<Pick<Merge<ResolvedFields<Options['defaultFields']>, ResolvedFields<T>>, keyof Type>> {
+      const seq = getSeq();
+      return resolveFields<Type, TransientFields, Options['defaultFields'], T>(
+        typeFieldNames,
+        seq,
+        defaultFieldsResolver,
+        inputFieldsResolver ?? ({} as T),
+      );
+    },
+    resetSequence() {
+      resetSequence(seqKey);
+    },
+  };
+}
diff --git a/src/field-resolver.ts b/src/field-resolver.ts
index de6509c..1e5dd83 100644
--- a/src/field-resolver.ts
+++ b/src/field-resolver.ts
@@ -31,12 +31,7 @@ export type DefaultFieldsResolver<TypeWithTransientFields> = {
   >;
 };
 /** The type of `inputFields` option of `build` method. */
-export type InputFieldsResolver<TypeWithTransientFields> = {
-  [FieldName in keyof TypeWithTransientFields]?: FieldResolver<
-    TypeWithTransientFields,
-    DeepReadonly<DeepOptional<TypeWithTransientFields>[FieldName]>
-  >;
-};
+export type InputFieldsResolver<TypeWithTransientFields> = Partial<DefaultFieldsResolver<TypeWithTransientFields>>;
 
 // eslint-disable-next-line @typescript-eslint/no-unused-vars
 export type ResolvedField<T extends FieldResolver<unknown, unknown>> = T extends FieldResolver<infer _, infer R>
diff --git a/src/index.ts b/src/index.ts
index e979e6a..8e48ac1 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,12 +1,6 @@
-import {
-  type ResolvedFields,
-  type DefaultFieldsResolver,
-  type InputFieldsResolver,
-  resolveFields,
-  lazy,
-} from './field-resolver.js';
-import { getSequenceCounter, resetSequence, resetAllSequence } from './sequence.js';
-import { type Merge } from './util.js';
+import { TypeFactoryDefineOptions, TypeFactoryInterface, defineTypeFactoryInternal } from './factory.js';
+import { lazy } from './field-resolver.js';
+import { resetAllSequence } from './sequence.js';
 
 export { resetAllSequence, lazy };
 
@@ -32,43 +26,14 @@ const UserFieldNames = ['id', 'firstName', 'lastName', 'fullName'] as const;
 
 // ---------- Book ----------
 
-export interface BookFactoryDefineOptions<TransientFields extends Record<string, unknown>> {
-  defaultFields: DefaultFieldsResolver<Book & TransientFields>;
-}
-export interface BookFactoryInterface<
+export type BookFactoryDefineOptions<TransientFields extends Record<string, unknown>> = TypeFactoryDefineOptions<
+  Book,
+  TransientFields
+>;
+export type BookFactoryInterface<
   TransientFields extends Record<string, unknown>,
   Options extends BookFactoryDefineOptions<TransientFields>,
-> {
-  build(): Promise<Pick<Merge<ResolvedFields<Options['defaultFields']>, ResolvedFields<{}>>, keyof Book>>;
-  build<const T extends InputFieldsResolver<Book & TransientFields>>(
-    inputFieldsResolver: T,
-  ): Promise<Pick<Merge<ResolvedFields<Options['defaultFields']>, ResolvedFields<T>>, keyof Book>>;
-  resetSequence(): void;
-}
-
-function defineBookFactoryInternal<
-  TransientFields extends Record<string, unknown>,
-  Options extends BookFactoryDefineOptions<TransientFields>,
->({ defaultFields: defaultFieldsResolver }: Options): BookFactoryInterface<TransientFields, Options> {
-  const seqKey = {};
-  const getSeq = () => getSequenceCounter(seqKey);
-  return {
-    async build<const T extends InputFieldsResolver<Book & TransientFields>>(
-      inputFieldsResolver?: T,
-    ): Promise<Pick<Merge<ResolvedFields<Options['defaultFields']>, ResolvedFields<T>>, keyof Book>> {
-      const seq = getSeq();
-      return resolveFields<Book, TransientFields, Options['defaultFields'], T>(
-        BookFieldNames,
-        seq,
-        defaultFieldsResolver,
-        inputFieldsResolver ?? ({} as T),
-      );
-    },
-    resetSequence() {
-      resetSequence(seqKey);
-    },
-  };
-}
+> = TypeFactoryInterface<Book, TransientFields, Options>;
 
 /**
  * Define factory for {@link Book} model.
@@ -79,48 +44,19 @@ function defineBookFactoryInternal<
 export function defineBookFactory<Options extends BookFactoryDefineOptions<{}>>(
   options: Options,
 ): BookFactoryInterface<{}, Options> {
-  return defineBookFactoryInternal(options);
+  return defineTypeFactoryInternal<Book, {}, Options>(BookFieldNames, options);
 }
 
 // ---------- Author ----------
 
-export interface AuthorFactoryDefineOptions<TransientFields extends Record<string, unknown>> {
-  defaultFields: DefaultFieldsResolver<Author & TransientFields>;
-}
-export interface AuthorFactoryInterface<
-  TransientFields extends Record<string, unknown>,
-  Options extends AuthorFactoryDefineOptions<TransientFields>,
-> {
-  build(): Promise<Pick<Merge<ResolvedFields<Options['defaultFields']>, ResolvedFields<{}>>, keyof Author>>;
-  build<const T extends InputFieldsResolver<Author & TransientFields>>(
-    inputFieldsResolver: T,
-  ): Promise<Pick<Merge<ResolvedFields<Options['defaultFields']>, ResolvedFields<T>>, keyof Author>>;
-  resetSequence(): void;
-}
-
-function defineAuthorFactoryInternal<
+export type AuthorFactoryDefineOptions<TransientFields extends Record<string, unknown>> = TypeFactoryDefineOptions<
+  Author,
+  TransientFields
+>;
+export type AuthorFactoryInterface<
   TransientFields extends Record<string, unknown>,
   Options extends AuthorFactoryDefineOptions<TransientFields>,
->({ defaultFields: defaultFieldsResolver }: Options): AuthorFactoryInterface<TransientFields, Options> {
-  const seqKey = {};
-  const getSeq = () => getSequenceCounter(seqKey);
-  return {
-    async build<const T extends InputFieldsResolver<Author & TransientFields>>(
-      inputFieldsResolver?: T,
-    ): Promise<Pick<Merge<ResolvedFields<Options['defaultFields']>, ResolvedFields<T>>, keyof Author>> {
-      const seq = getSeq();
-      return resolveFields<Author, TransientFields, Options['defaultFields'], T>(
-        AuthorFieldNames,
-        seq,
-        defaultFieldsResolver,
-        inputFieldsResolver ?? ({} as T),
-      );
-    },
-    resetSequence() {
-      resetSequence(seqKey);
-    },
-  };
-}
+> = TypeFactoryInterface<Author, TransientFields, Options>;
 
 /**
  * Define factory for {@link Author} model.
@@ -131,48 +67,19 @@ function defineAuthorFactoryInternal<
 export function defineAuthorFactory<Options extends AuthorFactoryDefineOptions<{}>>(
   options: Options,
 ): AuthorFactoryInterface<{}, Options> {
-  return defineAuthorFactoryInternal(options);
+  return defineTypeFactoryInternal<Author, {}, Options>(AuthorFieldNames, options);
 }
 
 // ---------- User ----------
 
-export interface UserFactoryDefineOptions<TransientFields extends Record<string, unknown>> {
-  defaultFields: DefaultFieldsResolver<User & TransientFields>;
-}
-export interface UserFactoryInterface<
+export type UserFactoryDefineOptions<TransientFields extends Record<string, unknown>> = TypeFactoryDefineOptions<
+  User,
+  TransientFields
+>;
+export type UserFactoryInterface<
   TransientFields extends Record<string, unknown>,
   Options extends UserFactoryDefineOptions<TransientFields>,
-> {
-  build(): Promise<Pick<Merge<ResolvedFields<Options['defaultFields']>, ResolvedFields<{}>>, keyof User>>;
-  build<const T extends InputFieldsResolver<User & TransientFields>>(
-    inputFieldsResolver: T,
-  ): Promise<Pick<Merge<ResolvedFields<Options['defaultFields']>, ResolvedFields<T>>, keyof User>>;
-  resetSequence(): void;
-}
-
-function defineUserFactoryInternal<
-  TransientFields extends Record<string, unknown>,
-  Options extends UserFactoryDefineOptions<TransientFields>,
->({ defaultFields: defaultFieldsResolver }: Options): UserFactoryInterface<TransientFields, Options> {
-  const seqKey = {};
-  const getSeq = () => getSequenceCounter(seqKey);
-  return {
-    async build<const T extends InputFieldsResolver<User & TransientFields>>(
-      inputFieldsResolver?: T,
-    ): Promise<Pick<Merge<ResolvedFields<Options['defaultFields']>, ResolvedFields<T>>, keyof User>> {
-      const seq = getSeq();
-      return resolveFields<User, TransientFields, Options['defaultFields'], T>(
-        UserFieldNames,
-        seq,
-        defaultFieldsResolver,
-        inputFieldsResolver ?? ({} as T),
-      );
-    },
-    resetSequence() {
-      resetSequence(seqKey);
-    },
-  };
-}
+> = TypeFactoryInterface<User, TransientFields, Options>;
 
 /**
  * Define factory for {@link User} model.
@@ -183,5 +90,5 @@ function defineUserFactoryInternal<
 export function defineUserFactory<Options extends UserFactoryDefineOptions<{}>>(
   options: Options,
 ): UserFactoryInterface<{}, Options> {
-  return defineUserFactoryInternal(options);
+  return defineTypeFactoryInternal<User, {}, Options>(UserFieldNames, options);
 }