A cutter, detachably attachable to a support member, includes a shaft made of a resin and has a smaller-diameter portion and a larger-diameter portion formed integrally with the shaft. The smaller-diameter portion is rotatably supported by a plurality of bearings. The larger-diameter portion has a proximal end surface abutting on an end surface of one of the bearings, with a result that the cutter is locked. The cutter includes a flat blade located on a distal end of the larger-diameter portion. The cutter includes a metal member which can be attracted by a magnet located on the support member. The metal member is located on a distal end of the smaller-diameter portion. Contact portions are located on axially differing portions of the shaft respectively and are capable of contacting the bearings respectively. Thinned parts are located between the contact portions and extend in the axial direction of the shaft.