法令翻訳 http://www.japaneselawtranslat...ky=&page=3

(外貨建取引の換算)
第五十七条の三  居住者が、外貨建取引（外国通貨で支払が行われる資産の販売及び購入、役務の提供、金銭の貸付け及び借入れその他の取引をいう。以下この条において同じ。）を行つた場合には、当該外貨建取引の金額の円換算額（外国通貨で表示された金額を本邦通貨表示の金額に換算した金額をいう。次項において同じ。）は当該外貨建取引を行つた時における外国為替の売買相場により換算した金額として、その者の各年分の各種所得の金額を計算するものとする。

Conversion of Transactions in a Foreign Currency
Article 57-3  If a Resident makes a transaction in a foreign currency (meaning that the Resident sells or purchases assets, provides services, borrows or lends monies, or conducts any other transaction involving payment in a foreign currency; hereinafter the same applies in this Article), the yen equivalent of that transaction (meaning the amount arrived at when the amount of the foreign-currency-denominated transaction is converted into a Japanese-currency-denominated amount; the same applies in the following paragraph) is the amount arrived at when the amount of the transaction is converted into yen based on the foreign exchange rate as of the time of the foreign-currency transaction, and this is used to calculate the Resident's Income in Each Class for the year.

不動産所得、事業所得、山林所得又は雑所得を生ずべき業務を行う居住者が、先物外国為替契約等（外貨建取引によつて取得し、又は発生する資産若しくは負債の金額の円換算額を確定させる契約として財務省令で定めるものをいう。以下この項において同じ。）により外貨建取引によつて取得し、又は発生する資産若しくは負債の金額の円換算額を確定させた場合において、当該先物外国為替契約等の締結の日においてその旨を財務省令で定めるところによりその者の当該業務に係る帳簿書類その他の財務省令で定める書類に記載したときは、当該資産又は負債については、当該円換算額をもつて、前項の規定により換算した金額として、その者の各年分の不動産所得の金額、事業所得の金額、山林所得の金額又は雑所得の金額を計算するものとする。

If a Resident conducting business that is meant to generate real property income, business income, timber income, or miscellaneous income fixes the yen equivalent of assets acquired or liabilities incurred through a transaction in a foreign currency based on a foreign exchange futures contract or similar contract (meaning what is provided for by Ministry of Finance Order as a contract that fixes the yen equivalent of assets acquired or liabilities incurred through a transaction in a foreign currency; hereinafter the same applies in this paragraph) and enters an indication of this, pursuant to Ministry of Finance Order, in the books and documents connected with the Resident's business or in any other document prescribed by Ministry of Finance Order, as on the day that the foreign exchange futures contract or similar contract is concluded, the yen equivalent of the assets or liabilities is treated as the amount arrived at via the conversion into yen pursuant to the preceding paragraph, and this is used to calculate the Resident's real property income, business income, timber income, or miscellaneous income for each year.

前項に定めるもののほか、外貨建取引の換算の特例その他前二項の規定の適用に関し必要な事項は、政令で定める。

Beyond what is prescribed in the preceding paragraph, Cabinet Order establishes special provisions on the conversion of transactions in a foreign currency and provides for other necessary particulars concerning the application of the provisions of the preceding two paragraphs.