Processing tank raising/lowering device and processing tank raising/lowering method for electrical discharge machine

In an electrical discharge machine, the top end of a vertically extending electrode is supported by an electrode holder so as to be able to be raised and lowered, the circumferential surface of the bottom end of said electrode is supported by an electrode guide that is disposed so as to be able to move vertically with respect to the electrode holder, and the position of said electrode guide is detected by a position detection unit. This electrical discharge machine is provided with a work-tank raising/lowering device. During electrical discharge machining, said work-tank raising/lowering device raises/lowers a work tank, in accordance with the electrode-guide position (Hg) detected by the position detection unit, so as to set the distance from a prescribed part of the work tank to the electrode-guide position (Hg) equal to a prescribed value (Ha).