A driver unit (21) of a vertically slidable type is constructed by mounting a staple cartridge (23) to a cartridge holder (22) so as to freely slide in a vertical direction, and by urging the staple cartridge (23) toward a clincher unit (1) opposed thereto by a spring (29). When a clincher sleeve (6) has descended to push the staple cartridge (23), the staple cartridge will move up and down in parallel, and accordingly, a relative angle of a clincher (7) to a driver plate (26) will not change.