English version comes after Japanese.
源泉徴収票は捨ててはいけません。12月か1月の給与支払い時についてきます。給与明細を捨てたとしても源泉徴収票は手元に残しておきましょう。今いらないよと思っても何年もたってから過去の収入を遡る場合もあります。
Japanese companies are required to issue "payment detail sheet" after the last payment of a year. We can find on one sheet how much we got paid in a year and how much tax and social securities we paid. We should keep this sheet in hand in case we need to show our income and deductions.