A thermal protecting circuit is provided for an alternator controller. The protecting circuit comprises a switching member switching on/off the alternator; a sensor sensing a temperature of the switching member; and a switching control circuit. This switching control circuit switches off the switching member when the temperature of the switching member exceeds a first predetermined temperature. This switching control circuit switches on the switching member when the temperature of the switching member is lower than a second predetermined temperature. The second predetermined temperature is set to be higher than the first predetermined temperature.