diff --git a/client/src/App.tsx b/client/src/App.tsx
index 75d944e..b1b1125 100644
--- a/client/src/App.tsx
+++ b/client/src/App.tsx
@@ -1,10 +1,38 @@
-import { BrowserRouter, Outlet, Route, Routes } from "react-router";
+import { BrowserRouter, Navigate, Outlet, Route, Routes, useParams } from "react-router";
 import SubmissionPage from "./pages/eventId/Submission.tsx";
 import HomePage from "./pages/Home.tsx";
 import LandingPage from "./pages/Landing.tsx";
 import NotFoundPage from "./pages/NotFound.tsx";
 import ProjectPage from "./pages/Project.tsx";
 
+/**
+ * Nano ID 形式の正規表現。
+ * 現在はハイフン・アンダースコアを含まないが、初期は含んでいたため URL の検証時は両方許容する。
+ */
+const NANOID_REGEX = /^[A-Za-z0-9_-]{21}$/;
+
+/**
+ * 旧パス /:eventId から新パス /e/:eventId へのリダイレクト。eventId は Nano ID 形式。
+ */
+function LegacyEventRedirect() {
+  const { eventId } = useParams();
+  if (!eventId || !NANOID_REGEX.test(eventId)) {
+    return <NotFoundPage />;
+  }
+  return <Navigate to={`/e/${eventId}`} replace />;
+}
+
+/**
+ * 旧パス /:eventId/edit から新パス /e/:eventId/edit へのリダイレクト。eventId は Nano ID 形式。
+ */
+function LegacyEventEditRedirect() {
+  const { eventId } = useParams();
+  if (!eventId || !NANOID_REGEX.test(eventId)) {
+    return <NotFoundPage />;
+  }
+  return <Navigate to={`/e/${eventId}/edit`} replace />;
+}
+
 export default function App() {
   return (
     <BrowserRouter>
@@ -12,11 +40,19 @@ export default function App() {
         <Route index element={<LandingPage />} />
         <Route path="home" element={<HomePage />} />
         <Route path="new" element={<ProjectPage />} />
-        <Route path=":eventId" element={<Outlet />}>
-          <Route index element={<SubmissionPage />} />
-          <Route path="edit" element={<ProjectPage />} />
-          <Route path="*" element={<NotFoundPage />} />
+
+        <Route path="e">
+          <Route path=":eventId" element={<Outlet />}>
+            <Route index element={<SubmissionPage />} />
+            <Route path="edit" element={<ProjectPage />} />
+            <Route path="*" element={<NotFoundPage />} />
+          </Route>
         </Route>
+
+        {/* /:eventId (旧形式)  を /e/:eventId (新形式) にリダイレクト */}
+        <Route path=":eventId" element={<LegacyEventRedirect />} />
+        <Route path=":eventId/edit" element={<LegacyEventEditRedirect />} />
+
         <Route path="*" element={<NotFoundPage />} />
       </Routes>
     </BrowserRouter>
diff --git a/client/src/pages/Home.tsx b/client/src/pages/Home.tsx
index 79700df..9c3c7b7 100644
--- a/client/src/pages/Home.tsx
+++ b/client/src/pages/Home.tsx
@@ -107,7 +107,7 @@ function ProjectDashboard({ involvedProjects }: { involvedProjects: BriefProject
 function ProjectCard({ project }: { project: BriefProject }) {
   return (
     <NavLink
-      to={`/${project.id}`}
+      to={`/e/${project.id}`}
       className={`group hover:-translate-y-1 relative block transform overflow-hidden rounded-xl border-l-4 bg-white shadow-lg transition-all duration-300 hover:shadow-xl ${project.isHost ? "border-primary" : "border-secondary"} focus:outline-none focus:ring-4 focus:ring-primary/20`}
       aria-label={`「${project.name}」の詳細を見る`}
     >
diff --git a/client/src/pages/Project.tsx b/client/src/pages/Project.tsx
index 86331ce..722ccf2 100644
--- a/client/src/pages/Project.tsx
+++ b/client/src/pages/Project.tsx
@@ -242,7 +242,7 @@ export default function ProjectPage() {
   useEffect(() => {
     if (!loading && project && !isHost) {
       if (eventId) {
-        navigate(`/${eventId}`);
+        navigate(`/e/${eventId}`);
       } else {
         navigate("/");
       }
@@ -613,7 +613,7 @@ export default function ProjectPage() {
               )}
               <div className="fixed bottom-0 left-0 flex max-h-18 w-full justify-between bg-white p-4 shadow-[0_-2px_8px_rgba(0,0,0,0.1)]">
                 {eventId ? (
-                  <NavLink to={`/${eventId}`} className="btn btn-outline btn-primary">
+                  <NavLink to={`/e/${eventId}`} className="btn btn-outline btn-primary">
                     日程調整に戻る
                   </NavLink>
                 ) : (
@@ -639,12 +639,12 @@ export default function ProjectPage() {
                 type="text"
                 disabled
                 className="input input-info w-full"
-                value={`${FRONTEND_ORIGIN}/${dialogStatus.projectId}`}
+                value={`${FRONTEND_ORIGIN}/e/${dialogStatus.projectId}`}
               />
               <button
                 type="button"
                 onClick={async () => {
-                  await navigator.clipboard.writeText(`${FRONTEND_ORIGIN}/${dialogStatus.projectId}`);
+                  await navigator.clipboard.writeText(`${FRONTEND_ORIGIN}/e/${dialogStatus.projectId}`);
                   setCopied(true);
                   setTimeout(() => {
                     setCopied(false);
@@ -657,7 +657,7 @@ export default function ProjectPage() {
               </button>
             </div>
             <div className="modal-action">
-              <NavLink className="btn btn-primary" to={`/${dialogStatus.projectId}`}>
+              <NavLink className="btn btn-primary" to={`/e/${dialogStatus.projectId}`}>
                 イベントへ
               </NavLink>
             </div>
diff --git a/client/src/pages/eventId/Submission.tsx b/client/src/pages/eventId/Submission.tsx
index a422e60..e199edf 100644
--- a/client/src/pages/eventId/Submission.tsx
+++ b/client/src/pages/eventId/Submission.tsx
@@ -210,7 +210,7 @@ export default function SubmissionPage() {
     <>
       <div className="flex h-[100dvh] flex-col">
         <Header />
-        {loading || !selectedParticipationOptionId ? (
+        {loading ? (
           <div className="flex w-full flex-1 items-center justify-center">
             <span className="loading loading-dots loading-md text-gray-400" />
           </div>
@@ -221,12 +221,16 @@ export default function SubmissionPage() {
               ホームに戻る
             </NavLink>
           </div>
+        ) : !selectedParticipationOptionId ? (
+          <div className="flex w-full flex-1 items-center justify-center">
+            <span className="loading loading-dots loading-md text-gray-400" />
+          </div>
         ) : (
           <div className="flex h-full flex-1 flex-col overflow-y-auto p-4">
             <div className="flex items-center justify-between">
               <h1 className="mb-2 font-bold text-2xl text-gray-800">{project.name} の日程調整</h1>
               {isHost && (
-                <NavLink to={`/${projectId}/edit`} className="btn btn-sm font-normal text-gray-600">
+                <NavLink to={`/e/${projectId}/edit`} className="btn btn-sm font-normal text-gray-600">
                   <HiOutlineCog />
                   イベント設定
                 </NavLink>
diff --git a/server/src/routes/projects.ts b/server/src/routes/projects.ts
index 89de447..948ce5d 100644
--- a/server/src/routes/projects.ts
+++ b/server/src/routes/projects.ts
@@ -1,13 +1,18 @@
 import { zValidator } from "@hono/zod-validator";
 import dotenv from "dotenv";
 import { Hono } from "hono";
-import { nanoid } from "nanoid";
+import { customAlphabet } from "nanoid";
 import { z } from "zod";
 import { editReqSchema, projectReqSchema, submitReqSchema } from "../../../common/validators.js";
 import { prisma } from "../main.js";
 
 dotenv.config();
 
+/**
+ * ハイフン・アンダースコアを含まない Nano ID 形式。
+ */
+const nanoid = customAlphabet("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789", 21);
+
 const projectIdParamsSchema = z.object({ projectId: z.string().length(21) });
 
 type AppVariables = {