Abstract: In a power supply circuit of a snowmobile in which an AC voltage of a magneto 51 is converted into a DC voltage to charge a battery 53 and electric power is supplied to an ECU 58 through a power relay 55 in response to turning on of a main switch 54 to inject fuel to an engine, a light relay 1 is provided to supply electric power from the battery 53 to electric loads such as headlights and the like and the ECU 58 judges a frequency of the AC voltage generated by the magneto 51 and control to turn on or off the light relay 1 in accordance with whether the frequency exceeds a predetermined value or not.