Katanas are not illegal in Japan, but their possession and sale are subject to strict regulations. The Sword and Firearms Control Law (刀砲刀剣類所持等取締法, Tōhō Tōkenrui Shoji-tō Torishimari Hō) governs the possession of swords in Japan.

Under this law, there are specific criteria and regulations regarding the ownership and sale of swords, including katanas. Genuine antique swords, particularly those recognized as Important Cultural Properties or National Treasures, are generally allowed for possession and sale, but they require appropriate licensing and registration.

However, the possession of non-traditional swords, replica swords, or modern-made swords without significant historical value may be restricted or prohibited under the law.

It's important to note that the specific regulations and requirements for owning and acquiring katanas can vary depending on factors such as the type of sword, its age, and other criteria. To ensure compliance with the regulations and to obtain accurate and up-to-date information, it is advisable to consult with Japanese government authorities, such as the local police or the Agency for Cultural Affairs. They can provide the necessary guidance on the possession and sale of katanas and help navigate the legal procedures and permits required.