Abstract: A blade is removably supported on one end of a shank which is axially movably disposed in a tubular holder. The shank has an axially extending slit terminating at the end thereof for supporting the blade therein and a tapered portion having an outside diameter decreasing gradually from the end of the shank. The blade is movable from one of its retracted, cutting and replacement positions to another with the movement of the shank. A sleeve is fitted about the shank adjacent to the end thereof and has one end facing the tapered portion thereof. A coiled spring is provided on the shank for urging one end of the sleeve into contact with the tapered portion of the shank. The sleeve is axially movable with the movement of the shank. The holder is provided on its inner surface adjacent to the sleeve for preventing the movement of the sleeve upon movement of the blade to its replacement position.