There is provided a starter system for an internal combustion engine, which is capable of preventing wear of a brush of an electric motor ascribable to the combined use of the electric motor with a hydraulic actuator and the resulting increase in the rotational resistance due to friction of the brush, as well as capable of using one of the hydraulic actuator and the electric motor without difficulty even when the other is disabled. The hydraulic motor is driven by hydraulic pressure. A rotational shaft of the hydraulic motor is driven for rotation by the hydraulic motor. A rotational shaft of an electric motor extends in parallel with the rotational shaft of the hydraulic motor, and is driven for rotation by the electric motor. A ring gear rotates in unison with the crankshaft. A pinion gear is brought into meshing engagement with the ring gear, for starting the engine. An output shaft is connected to the pinion gear. A first one-way clutch connects the rotational shaft of the hydraulic motor and the output shaft to each other in a disconnectable manner, for transmitting rotation of the hydraulic motor to the output shaft. A second one-way clutch connects the rotational shaft of the electric motor and the output shaft to each other in a disconnectable manner, for transmitting rotation of the rotational shaft of the electric motor to the output shaft.