To achieve the cutting with the overlapping margin of constant width, to facilitate the cutting by keeping the prescribed clearance from the reentrant angle line, and to prevent a waterproof sheet on the lower side from being cut.

A cutting blade 2 is fitted to a supporting plate 1 provided with a press roller (a) and a rolling roller (b) to press a waterproof sheet Y along the reentrant angle line X. The cutting blade 2 pierces both sides of the supporting plate 1, the press roller (a) is provided in an inclined manner, a guide plate 3 is provided on an edge part of the supporting plate, the press roller and the guide plate or the cutting blade are movable, the tip of the cutting blade is of sickle shape, and the press roller (a) is fitted through a spring or a buffer.