Z:gnu-www-ja-gpl-howto--7f60a9-There is no legal requirement/en

There is no legal requirement to register your copyright with anyone; simply distributing the program makes it copyrighted. However, it is a very good idea to register the copyright with the US Registry of Copyrights, because that puts you in a stronger position against anyone who violates the license in the US. Most other countries have no system of copyright registration.