To break and remove the tip section of an abraded file at a break groove and use up the file to the last without causing failing to break by forming break grooves on the substrate of the file fixed with abrasive grains on an operating surface.

This flat file is constituted of an operating surface 1 and a grip section 2, and abrasive grains are fixed to the operating surface 1. Break grooves 3 are formed on both faces of the operating surface 1, and the operating surface 1 is divided into multiple blocks by the break grooves 3. Only the foremost block of the file is used for thread chamfering. When the foremost block is abraded, the file is broken at the break groove 3 between the foremost block and the second block to remove the foremost block, and the new foremost block having a new operating surface is used to continue the finishing work. The file can be used up with no waste to the last block without causing failing to break.