A starter motor comprising a pinion (3) which engages at an inner circumference of a gear portion thereof with an outer circumference of an output rotary shaft (4), and a cylindrical member (15) integrally disposed behind the pinion. A front end portion of a clutch inner member (9b) of an over-running clutch unit (9) is in sliding abutment with an outer circumferential surface of the cylindrical member, and the inner circumferential surface of the cylinder member is in splined engagement with the output rotary shaft.