Motor starting device

Provided is an engine starting device which is capable of simply a pinion gear in mesh with a ring gear to be engaged in order to start a motor again, while the ring gear is rotating, regardless of whether the rotation of the gear ring is done in a forward direction or a reverse direction even if an rpm difference between the ring gear and the pinion gear is large. The engine starting device includes a pinion unit (30) which is connected to an output shaft side of the starter motor by means of a splined shaft to slide in an axial direction, wherein said pinion unit (30) includes an overrunning clutch which during rotation in the same direction as that of rotation of the motor by rotating, and a pushing device (40, 50, 60) for moving the pinion unit (30) engages in a position in which the pinion unit (30) in mesh with a ring gear (100). A pinion gear (34) of the pinion unit (30) is operable in the axial direction as a single body, and includes a tapered portion (34d) having a curved shape along a tooth surface (34c) on a no torque transmitting surface side and a long length in the axial direction wherein the tapered portion (34d) on a distal side of the gear rim on the end portion of the tooth surface is not on the torque transmitting surface side, is provided (34c).