diff --git a/main.js b/main.js
index 93c5127..7d14f86 100644
--- a/main.js
+++ b/main.js
@@ -12890,6 +12890,7 @@ function validateStandaloneDomains(index, warnings) {
       });
     }
   }
+  validateStandaloneDomainParents(entriesByDomainId, warnings);
   for (const entries of entriesByDomainId.values()) {
     if (entries.length < 2) {
       continue;
@@ -12918,6 +12919,24 @@ function validateStandaloneDomains(index, warnings) {
     compareStandaloneDomainFields(sortedEntries, warnings);
   }
 }
+function validateStandaloneDomainParents(entriesByDomainId, warnings) {
+  for (const entries of entriesByDomainId.values()) {
+    for (const entry of entries) {
+      const parent = entry.domain.parent?.trim();
+      if (!parent || parent === entry.domain.id || entriesByDomainId.has(parent)) {
+        continue;
+      }
+      warnings.push({
+        code: "unresolved-reference",
+        message: formatDomainParentUnknownMessage(parent),
+        severity: "warning",
+        path: entry.path,
+        field: "Domains.parent",
+        context: { rowIndex: entry.domain.rowIndex + 1 }
+      });
+    }
+  }
+}
 function compareStandaloneDomainFields(entries, warnings) {
   for (const field of STANDALONE_DOMAIN_CANONICAL_FIELDS) {
     const values = new Set(entries.map((entry) => entry.domain[field]?.trim() ?? ""));
@@ -13326,6 +13345,7 @@ function ensureVaultValidation(index) {
     return;
   }
   clearVaultValidationWarnings(index);
+  clearDomainParentNotDefinedWarnings(index);
   for (const warning of validateVaultIndex(index)) {
     pushWarning(index.warningsByFilePath, warning.path ?? "vault", {
       ...warning,
@@ -13676,6 +13696,24 @@ function clearVaultValidationWarnings(index) {
     );
   }
 }
+function clearDomainParentNotDefinedWarnings(index) {
+  for (const [path2, warnings] of Object.entries(index.warningsByFilePath)) {
+    const model = index.modelsByFilePath[path2];
+    if (model?.fileType !== "domains") {
+      continue;
+    }
+    index.warningsByFilePath[path2] = warnings.filter(
+      (warning) => !getDomainParentNotDefinedValue(warning)
+    );
+  }
+}
+function getDomainParentNotDefinedValue(warning) {
+  if (warning.code !== "unresolved-reference" || warning.field !== "Domains.parent") {
+    return null;
+  }
+  const match = warning.message.match(/^Domain parent "([^"]+)" is not defined\.$/);
+  return match?.[1] ?? null;
+}
 function getDuplicateModelKey(model) {
   if (model.fileType === "markdown") {
     return null;
diff --git a/src/core/validator.ts b/src/core/validator.ts
index 64c9e18..4f27fa6 100644
--- a/src/core/validator.ts
+++ b/src/core/validator.ts
@@ -14,6 +14,7 @@ import {
   formatDfdObjectUnknownLocalDomainMessage,
   formatDfdLocalDomainFieldMismatchMessage,
   formatDfdLocalDomainMissingSharedMessage,
+  formatDomainParentUnknownMessage,
   formatStandaloneDomainDuplicateMessage,
   formatStandaloneDomainFieldConflictMessage
 } from "./domain-diagnostics";
@@ -127,6 +128,8 @@ function validateStandaloneDomains(
     }
   }
 
+  validateStandaloneDomainParents(entriesByDomainId, warnings);
+
   for (const entries of entriesByDomainId.values()) {
     if (entries.length < 2) {
       continue;
@@ -159,6 +162,28 @@ function validateStandaloneDomains(
   }
 }
 
+function validateStandaloneDomainParents(
+  entriesByDomainId: Map<string, Array<{ domain: DomainEntry; path: string }>>,
+  warnings: ValidationWarning[]
+): void {
+  for (const entries of entriesByDomainId.values()) {
+    for (const entry of entries) {
+      const parent = entry.domain.parent?.trim();
+      if (!parent || parent === entry.domain.id || entriesByDomainId.has(parent)) {
+        continue;
+      }
+      warnings.push({
+        code: "unresolved-reference",
+        message: formatDomainParentUnknownMessage(parent),
+        severity: "warning",
+        path: entry.path,
+        field: "Domains.parent",
+        context: { rowIndex: entry.domain.rowIndex + 1 }
+      });
+    }
+  }
+}
+
 function compareStandaloneDomainFields(
   entries: Array<{ domain: DomainEntry; path: string }>,
   warnings: ValidationWarning[]
diff --git a/src/core/vault-index.ts b/src/core/vault-index.ts
index 5edefa9..05fa3f3 100644
--- a/src/core/vault-index.ts
+++ b/src/core/vault-index.ts
@@ -182,6 +182,7 @@ export function ensureVaultValidation(index: ModelingVaultIndex): void {
   }
 
   clearVaultValidationWarnings(index);
+  clearDomainParentNotDefinedWarnings(index);
   for (const warning of validateVaultIndex(index)) {
     pushWarning(index.warningsByFilePath, warning.path ?? "vault", {
       ...warning,
@@ -599,6 +600,27 @@ function clearVaultValidationWarnings(index: ModelingVaultIndex): void {
   }
 }
 
+function clearDomainParentNotDefinedWarnings(index: ModelingVaultIndex): void {
+  for (const [path, warnings] of Object.entries(index.warningsByFilePath)) {
+    const model = index.modelsByFilePath[path];
+    if (model?.fileType !== "domains") {
+      continue;
+    }
+
+    index.warningsByFilePath[path] = warnings.filter(
+      (warning) => !getDomainParentNotDefinedValue(warning)
+    );
+  }
+}
+
+function getDomainParentNotDefinedValue(warning: ValidationWarning): string | null {
+  if (warning.code !== "unresolved-reference" || warning.field !== "Domains.parent") {
+    return null;
+  }
+  const match = warning.message.match(/^Domain parent "([^"]+)" is not defined\.$/);
+  return match?.[1] ?? null;
+}
+
 function getDuplicateModelKey(
   model: ParsedFileModel
 ): { key: string; id: string } | null {
diff --git a/test/domains-model.test.mjs b/test/domains-model.test.mjs
index b22ab0f..be370dd 100644
--- a/test/domains-model.test.mjs
+++ b/test/domains-model.test.mjs
@@ -534,6 +534,24 @@ test("localizes standalone Domain diagnostics", () => {
   );
 });
 
+test("allows standalone Domains parent from another Domains file", () => {
+  const index = buildVaultIndex([
+    domainsFile("COUNTRY_EXAMPLE.md", "COUNTRY_EXAMPLE", "| software_develop | Software develop | business | | Shared parent |"),
+    domainsFile("COUNTRY_EXAMPLE_1.md", "COUNTRY_EXAMPLE_1", [
+      "| software_develop_eigyo_1 | Sales 1 | team | software_develop | External parent |",
+      "| software_develop_kaihatsu_1 | Development 1 | team | software_develop | External parent |"
+    ].join("\n"))
+  ]);
+
+  const messages = (index.warningsByFilePath["COUNTRY_EXAMPLE_1.md"] ?? []).map(
+    (warning) => warning.message
+  );
+  assert.equal(
+    messages.includes("Domain parent \"software_develop\" is not defined."),
+    false
+  );
+});
+
 test("validates duplicate standalone Domains across files", () => {
   const index = buildVaultIndex([
     {
@@ -1498,6 +1516,10 @@ id: DOMAIN-DIAGRAM-LOGISTICS
     ["company", "logistics", "warehouse", "team_1"]
   );
   assert.equal(resolved.warnings.length, 0);
+  const sourceMessages = (index.warningsByFilePath["model/domains/DOMAINS-WAREHOUSE.md"] ?? []).map(
+    (warning) => warning.message
+  );
+  assert.equal(sourceMessages.includes("Domain parent \"logistics\" is not defined."), false);
   assert.match(buildDomainMindmapMermaid(resolved.domains), /会社全体（organization）/);
   assert.match(buildDomainHierarchyMermaid(resolved.domains), /subgraph domain_company/);
   assert.match(