Enter it as a Pension.

Foreign Social Security Pensions
Most income tax treaties have special rules for social security payments. In many cases, foreign social security payments are taxable by the country making the payments. Unless specified otherwise in an income tax treaty, foreign social security pensions are generally taxed as if they were foreign pensions or foreign annuities. Unless a tax treaty allows it (see, e.g., the USA-Canada treaty), they are not eligible for exclusion from taxable income the way a U.S. social security pension might be.


https://www.irs.gov/businesses/the-t...-distributions



A quick glance seems to indicate only German and Canadian Social Security are eligible for the US tax-treatment of Social Security of being only partial taxation.

Canadian or German social security benefits paid to U.S. residents.

Under income tax treaties with Canada and Germany, social security benefits paid by those countries to U.S. residents are treated for U.S. income tax purposes as if they were paid under the social security legislation of the United States. If you receive social security benefits from Canada or Germany, include them on line 1 of Worksheet 1, shown later.


https://www.irs.gov/publications/p91...blink100097884



There doesn't seem to be any special rules for Japanese Social Security:

https://www.irs.gov/pub/irs-trty/japante04.pdf#page=71