A starter includes a movable pinion cylinder mounted on a rotary output shaft so that the movable pinion cylinder is slidable on the rotary output shaft a by predetermined distance together with an overrunning clutch. The movable pinion cylinder has the front end portion formed into a pinion which is engaged with an engine ring gear, and the rear end portion formed into a clutch inner forming a part of the overrunning clutch. The starter further includes a cylindrical cap connected to the front end of the rotary output shaft to cover the front end portion of the rotary output shaft, a protrusion extended from the center of the end face of the cap toward the end face of the rotary output shaft, the protrusion having an air hole extended along the axial line thereof, and a hole formed in the end face of the rotary output shaft at the center to receive the protrusion.