background.js chrome.runtime.onMessage.addEventListener((req, sender, sendRes) => { setTimeout(() => { // ここまで来てるのに sendRes("元気でーす"); }, 100); }); contents_script.js chrome.runtime.sendMessage(null, {msg:"元気ですか?"}, (res) => { window.alert("返事:" + res); // ここに来ない }); 原因 stackoverflow.com chrome.runtime - Google Chrome sendResponse Function to call (at most once) when you have a response. The argume