Abstract: A portable motor-driven cutter for cutting off a sheet material includes an eccentric shaft rotatably supported by a gear case and a gear cover, an L-shaped driver arm having an upper end rotatably fitted over an eccentric portion of the eccentric shaft, a cutting blade detachably mounted on the lower end of the driver arm, and a scribing blade detachably mounted on a base on which the gear case is mounted. When the eccentric shaft is rotated by a motor, the cutting blade is moved in an eccentric pattern to cut off a sheet material along a straight line scribed by the scribing blade. The cutting blade and the scribing blade are produced from a single break-off disposable cutter blade.