これで上手くいった。
# coding:utf-8 require 'gmail' require 'base64' gmail = Gmail.connect("YOUR_GMAIL_ACCOUNT","YOUR_PASSWORD") mails = gmail.mailbox('YOUR_GMAIL_LABEL').all latest = mails.first subject = latest.subject subject = subject.gsub("=?ISO-2022-JP?B?","").gsub(/\?\=$/,"") subject = Base64.decode64(subject).force_encoding('ISO-2022-JP').encode('UTF-8') puts subject