請問香港手機有 3 頻、4頻，有什麼頻可以適合日本用？

如果你用緊 3G 服務及手機. 大部份去到日本都可以用 ka la. 打番去你既網絡商. check 清楚價錢啦. 因都幾貴 ka. 如果唔想咁貴, 想有 D 預算先. 可考慮租機服務 lor.
http://www.japanphone-rental.com/rental-charges-cn.htm

Actually, most of the 3G phones can be used in Japan. However, you should check with your service provider before. For my understanding, 3 and One2Free are service providers (I am not sure others, you can check) which can support the phone service in Japan if you have a 3G phone. But you should activate your oversea service first, please check with the shop for detail.