PURPOSE: To reduce the load on a starting motor by starting the starting motor by turning a clutch off in a state that an engine is not started, on the other hand, by prohibiting the motion of the start motor in a state of the engine operating.

CONSTITUTION: Since the output voltage of a generator 32 is sufficiently low before starting an engine, the normally-closed contact 46a of a start prohibiting relay 46 is closed, and when a clutch is turned off with a clutch lever gripped, a clutch switch 38 is closed, and a starter relay 36 is excited, and a contact 36a is closed, and electric current flows in a starting motor 28, resulting in the start of the engine. When the engine starts, the output voltage of the generator 32 is increased, and the voltage between both terminals of the resistor 44 of a start detection circuit 40 is increased, and the contact 46a of the start prohibiting relay 46 is opened, cutting off the current to the starting motor 28. Since the clutch is thus turned off during the start of the engine, the load on the starting motor can be reduced.