一時所得とは、利子所得、配当所得、不動産所得、事業所得、給与所得、退職所得、山林所得及び譲渡所得以外の所得のうち、営利を目的とする継続的行為から生じた所得以外の一時の所得で労務その他の役務又は資産の譲渡の対価としての性質を有しないものをいう。
Occasional income means income outside of interest income, dividend income, real property income, business income, salary income, retirement income, timber income, and capital gains; which does not constitute income from one-time payments arising from continuous activities undertaken on a commercial basis; and which does not have the nature of compensation for work or other services or for the transfer of assets.
一時所得の金額は、その年中の一時所得に係る総収入金額からその収入を得るために支出した金額（その収入を生じた行為をするため、又はその収入を生じた原因の発生に伴い直接要した金額に限る。）の合計額を控除し、その残額から一時所得の特別控除額を控除した金額とする。
Occasional income is the amount arrived at when the amounts expended to gain the revenue arising from occasional income for the year are added together (but only amounts that were directly needed to bring about the undertaking of acts that generated the revenue or to provoke the occurrence of causes that generated the revenue), the sum total thereof is deducted from gross revenues constituting occasional income in the relevant year, and the special occasional income deduction is applied against the net balance.
The special occasional income deduction provided for in the preceding paragraph is 500,000 yen (or the net balance provided for in that paragraph, if this is less than 500,000 yen).