A starter motor is adapted to transmit through an overrunning clutch device a rotating force of an armature to a rotary output shaft to which a pinion is fitted by means of a linear spline structure so that the pinion is engageable with a ring gear connected to an engine. The rotary output shaft has a shoulder portion between a helical spline portion, which is engaged with a helical spline which in turn is formed at the inner circumference of the front part of a clutch inner member in the overrunning clutch device, and a linear spline portion to which the pinion is fitted, and a spring is disposed inside the inner diameter of the heical spline of the clutch inner member and between the helical spline of the rotary output shaft and the rear end of the pinion so that the pinion is pushed forwardly wherein the shoulder portion has a diamter smaller than the diameter of the root of the helical spline of the rotary output shaft.