A pinion gear unit includes a rotation member rotating according to the rotation of a output shaft. A first pinion gear fit to the rotation member through a first serration unit rotates with, and also moves with the rotation member if the pinion gear unit moves towards an engagement location with an engine starting gear. A second pinion gear is fit to the rotation member adjacent to the first pinion gear through a second serration unit, rotates with and moves with the rotation member if the pinion gear unit further moves towards the engagement location, and engages with the starting gear later than the first pinion gear, where the first pinion gear engages with the engine starting gear. Fitting is performed by the first and second serration units so that phases of teeth of the first and second pinion gears in a circumferential direction correspond with each other.