A starter for an internal combustion engine includes a power transmission mechanism. The power transmission mechanism has an outer race plate fastened to a crankshaft, and a ring gear coupled to a rotational shaft of an electric motor for starting the engine. The power transmission mechanism further has a one-way clutch for permitting torque transmission only from the ring gear to the outer race plate, and two sealing members. Lubricant is supplied from the cylinder block to the interior of the power transmission mechanism for lubricating the power transmission mechanism. The outer race plate has a groove on its contact surface of a contact portion between the crankshaft and the outer race plate. The groove extends in the entire circumferential direction of the crankshaft. This maintains the coupling strength between the outer race plate and the crankshaft, while enhancing the sealing performance of the power transmission mechanism.