If you have a domicile in Japan or have a residence in Japan continuously for one year or more, you are considered a resident in Japan for tax purposes.
The amount paid under Japanese social security laws, such as Health Insurance, Nursing Insurance and Social Pension, are deducted from your taxable income.
In the following case, the amount paid for social security are also deductable from your income.
･The amount you paid for the spouse and relatives who made a living from your income, or who lived on common living expenses at the time.
･The past premium paid in this tax year.
･Within one year pre-paid premium.
The spouse and relatives who made a living from your income, or who lived on common living expenses are determined at the timing of paying premium, not the end of tax year.
Even if your son left your house and become economically independent after your paying his premium, the amount paid is deductable from your income.