diff --git a/src/field-resolver.ts b/src/field-resolver.ts
index afc4ee3..eced1dd 100644
--- a/src/field-resolver.ts
+++ b/src/field-resolver.ts
@@ -2,6 +2,7 @@ import { DeepOptional, DeepReadonly, Merge } from './util.js';
 
 export type FieldResolverOptions<Type> = {
   seq: number;
+  // TODO: transientFieldsResolver 由来の field name も受け取れるように。FieldResolverOptions<Type, TransientFieldNames> に拡張する必要があるのでは。
   get: <FieldName extends keyof Type>(fieldName: FieldName) => Promise<Type[FieldName]>;
 };
 
@@ -23,6 +24,8 @@ export type FieldResolver<Type, Field> = Field | Lazy<Type, Field>;
 export type DefaultFieldsResolver<Type> = {
   [FieldName in keyof Type]: FieldResolver<Type, DeepReadonly<DeepOptional<Type>[FieldName]>>;
 };
+/** The type of `transientFields` option of `defineFactory` function. */
+export type TransientFieldsResolver<Type> = Record<string, FieldResolver<Type, unknown>>;
 /** The type of `inputFields` option of `build` method. */
 export type InputFieldsResolver<Type> = {
   [FieldName in keyof Type]?: FieldResolver<Type, DeepReadonly<DeepOptional<Type>[FieldName]>>;
@@ -40,10 +43,12 @@ export type ResolvedFields<FieldsResolver extends Record<string, FieldResolver<u
 export async function resolveFields<
   Type extends Record<string, unknown>,
   _DefaultFieldsResolver extends DefaultFieldsResolver<Type> = DefaultFieldsResolver<Type>,
+  _TransientFieldsResolver extends TransientFieldsResolver<Type> = TransientFieldsResolver<Type>,
   _InputFieldsResolver extends InputFieldsResolver<Type> = InputFieldsResolver<Type>,
 >(
   seq: number,
   defaultFieldsResolver: _DefaultFieldsResolver,
+  transientFieldsResolver: _TransientFieldsResolver,
   inputFieldsResolver: _InputFieldsResolver,
 ): Promise<Merge<ResolvedFields<_DefaultFieldsResolver>, ResolvedFields<_InputFieldsResolver>>> {
   // eslint-disable-next-line @typescript-eslint/no-explicit-any -- Use any type as it is impossible to match types.
@@ -61,7 +66,7 @@ export async function resolveFields<
   }
 
   async function resolveFieldAndUpdateCache<FieldName extends keyof Type>(
-    fieldName: FieldName,
+    fieldName: FieldName, // TODO: transientFieldsResolver 由来の field name も受け取れるように
   ): Promise<Type[FieldName]> {
     if (fieldName in fields) return fields[fieldName];
 
diff --git a/src/index.ts b/src/index.ts
index 96fb2e4..88151e7 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,6 +1,7 @@
 import {
   type ResolvedFields,
   type DefaultFieldsResolver,
+  type TransientFieldsResolver,
   type InputFieldsResolver,
   resolveFields,
   lazy,
@@ -32,6 +33,7 @@ export type User = {
 
 interface BookFactoryDefineOptions {
   defaultFields: DefaultFieldsResolver<Book>;
+  transientFields?: TransientFieldsResolver<Book>;
 }
 interface BookFactoryInterface<TOptions extends BookFactoryDefineOptions> {
   build(): Promise<ResolvedFields<TOptions['defaultFields']>>;
@@ -44,6 +46,7 @@ interface BookFactoryInterface<TOptions extends BookFactoryDefineOptions> {
 
 function defineBookFactoryInternal<TOptions extends BookFactoryDefineOptions>({
   defaultFields: defaultFieldsResolver,
+  transientFields: transientFieldsResolver,
 }: TOptions): BookFactoryInterface<TOptions> {
   const seqKey = {};
   const getSeq = () => getSequenceCounter(seqKey);
@@ -52,7 +55,7 @@ function defineBookFactoryInternal<TOptions extends BookFactoryDefineOptions>({
       inputFieldsResolver?: T,
     ): Promise<Merge<ResolvedFields<TOptions['defaultFields']>, ResolvedFields<T>>> {
       const seq = getSeq();
-      return resolveFields(seq, defaultFieldsResolver, inputFieldsResolver ?? ({} as T));
+      return resolveFields(seq, defaultFieldsResolver, transientFieldsResolver ?? {}, inputFieldsResolver ?? ({} as T));
     },
     resetSequence() {
       resetSequence(seqKey);
@@ -76,6 +79,7 @@ export function defineBookFactory<TOptions extends BookFactoryDefineOptions>(
 
 interface AuthorFactoryDefineOptions {
   defaultFields: DefaultFieldsResolver<Author>;
+  transientFields?: TransientFieldsResolver<Author>;
 }
 interface AuthorFactoryInterface<TOptions extends AuthorFactoryDefineOptions> {
   build(): Promise<ResolvedFields<TOptions['defaultFields']>>;
@@ -88,6 +92,7 @@ interface AuthorFactoryInterface<TOptions extends AuthorFactoryDefineOptions> {
 
 function defineAuthorFactoryInternal<TOptions extends AuthorFactoryDefineOptions>({
   defaultFields: defaultFieldsResolver,
+  transientFields: transientFieldsResolver,
 }: TOptions): AuthorFactoryInterface<TOptions> {
   const seqKey = {};
   const getSeq = () => getSequenceCounter(seqKey);
@@ -96,7 +101,7 @@ function defineAuthorFactoryInternal<TOptions extends AuthorFactoryDefineOptions
       inputFieldsResolver?: T,
     ): Promise<Merge<ResolvedFields<TOptions['defaultFields']>, ResolvedFields<T>>> {
       const seq = getSeq();
-      return resolveFields(seq, defaultFieldsResolver, inputFieldsResolver ?? ({} as T));
+      return resolveFields(seq, defaultFieldsResolver, transientFieldsResolver ?? {}, inputFieldsResolver ?? ({} as T));
     },
     resetSequence() {
       resetSequence(seqKey);
@@ -120,6 +125,7 @@ export function defineAuthorFactory<TOptions extends AuthorFactoryDefineOptions>
 
 interface UserFactoryDefineOptions {
   defaultFields: DefaultFieldsResolver<User>;
+  transientFields?: TransientFieldsResolver<User>;
 }
 interface UserFactoryInterface<TOptions extends UserFactoryDefineOptions> {
   build(): Promise<ResolvedFields<TOptions['defaultFields']>>;
@@ -132,6 +138,7 @@ interface UserFactoryInterface<TOptions extends UserFactoryDefineOptions> {
 
 function defineUserFactoryInternal<TOptions extends UserFactoryDefineOptions>({
   defaultFields: defaultFieldsResolver,
+  transientFields: transientFieldsResolver,
 }: TOptions): UserFactoryInterface<TOptions> {
   const seqKey = {};
   const getSeq = () => getSequenceCounter(seqKey);
@@ -140,7 +147,7 @@ function defineUserFactoryInternal<TOptions extends UserFactoryDefineOptions>({
       inputFieldsResolver?: T,
     ): Promise<Merge<ResolvedFields<TOptions['defaultFields']>, ResolvedFields<T>>> {
       const seq = getSeq();
-      return resolveFields(seq, defaultFieldsResolver, inputFieldsResolver ?? ({} as T));
+      return resolveFields(seq, defaultFieldsResolver, transientFieldsResolver ?? {}, inputFieldsResolver ?? ({} as T));
     },
     resetSequence() {
       resetSequence(seqKey);