There are 2 ways to manage a private rental:
1. Obtain a rental accommodation business license, or
2. If the room is within Japan's National Strategic Special Zones, you can take advantage of Special Zone Law, Article 13 (Exceptions to Hotel Management Law) to receive special clearance from your municipality. In this case, you won't need a license. It can be difficult for regular citizens to attain a rental accommodation business license due to the many codes and standards that must be met, such as building standard laws and fire service laws.