A motor-operated stapler is disclosed which, when a bundle of sheets to be stapled is thin, prevents staple leg portions once inserted through the bundle of sheets from again piercing through the sheets bundle and which, in the case of a thick bundle of sheets, bends staple leg portions sufficiently to staple the bundle of sheets to a satisfactory extent. In the motor-operated stapler, a clincher base which carries a bundle of sheets thereon is supported by a housing of a magazine which pushes out a U-shaped staple downward, the bundle of sheets is pinched by both the clincher base and the magazine, the U-shaped staple is pushed out from the magazine into and through the bundle of sheets, and a pair of leg portions of the staple thus projected from the bundle of sheets are bent by a pair of clinchers installed on the clincher base.