diff --git a/src/components/downloadModal.tsx b/src/components/downloadModal.tsx
index 76e66296..9d3b2c92 100644
--- a/src/components/downloadModal.tsx
+++ b/src/components/downloadModal.tsx
@@ -1,5 +1,5 @@
 import { Link, graphql, useStaticQuery } from "gatsby"
-import React, { useState } from "react"
+import React, { useEffect, useState } from "react"
 import { APP_VERSION } from "../constants"
 import DownloadModalSelecter from "./downloadModalSelecter"
 
@@ -133,6 +133,17 @@ export const DownloadModal: React.FC<{
     ? selectedPackage
     : packageAvailables[selectedOs][selectedOrDefaultMode]![0]
 
+  useEffect(() => {
+    const userAgent = window.navigator.userAgent
+    if (userAgent.includes("Windows")) {
+      setSelectedOs("Windows")
+    } else if (userAgent.includes("Mac")) {
+      setSelectedOs("Mac")
+    } else if (userAgent.includes("Linux")) {
+      setSelectedOs("Linux")
+    }
+  }, [])
+
   const selectOs = (os: OsType) => {
     setSelectedOs(os)
     // 変更先のOSで選択できないモードの場合、最初のモードを選択する