If you aren’t a Japanese national and have lived in Japan for less than 5 years in the past 10 years, you are classified as a “Non-permanent resident” under Japanese income tax law.
Non –permanent residents are subject to Japanese income tax for the following types of income:
・Income generated from business operations in Japan
・Proceeds of sale or rent generated from real property located in Japan
・Certain types of interest (i.e. interests received on Japanese Government Bonds, Japanese Local Government Bonds, loans rendered for the business operation in Japan etc)
・Certain types of dividend (i.e. dividends received from Japanese companies)
・Royalties received for certain rights related to businesses operated in Japan
・Salaries received for services rendered in Japan (salaries for services rendered outside of Japan are subject to Japanese tax if such salary is remitted to Japan)
・Other revenues received in Japan or remitted to Japan from overseas
In your case, you don’t have to file rental income from an apartment in the USA since it does not fall into any of the above categories.