If you are planning to move house within Japan, go to your nearest Post Office and ask for a mail forwarding (tenso todoke ) form. After you submit the form (again, to any Post Office), Japan Post will start redirecting your mail from the date you have indicated on the form. Your mail will be redirected for one year.

You can request mail forwarding online, but you need to be able to read Japanese and have a credit card in your name (the service is free, but the credit card is required to verify your identity).