To solve the problem of wiring and a starter motor by preparing a low-voltage circuit and a high-voltage circuit, when responding to the requirement of operating a starter motor for a low-voltage power supply and a high- voltage power supply in common.

An armature 10 and a shunt coil 12 having a number of turns more than a series coil 11 are connected in parallel. When a low voltage is impressed to the starter motor 4, a large current flows to the series coil 11 with the small number of turns while a current flowing through the shunt coil 12 with the large number of turns is small. This constitution can reduce a magnetic field in a stator side so as to secure a rotation speed necessary for impressing the low voltage. On the contrary, when the high voltage is impressed, the high voltage is impressed to the shunt coil 12 with the large number of turns so as to generate a large magnetic field. This constitution can enlarge the magnetic field of the stator side so as to eliminate failure, or excessive rotation of the starter motor and obtain an appropriate rotation speed. The addition of the shunt coil 12 thus enables the sharing use.