To avoid malfunctioning of the rotation control of a fan motor and improve the safety and reliability thereof, by a method wherein a sensor circuit construction which does not rely upon only a revolution sensor attached to the fan motor is provided.

Current detection resistors R3 and R4 detect current values when fan motors 3 and 4 rotate and supply them to a protection circuit unit 10. When the fan motors 3 and 4 rotate, Zener diodes ZD1 and ZD2 in the protection circuit unit 10 are turned on and a thyristor SCR and a transistor Q are turned on. The collector of the transistor Q is connected to the input terminal 11e of a microcomputer 11, and binary signals in accordance with the current values from the current detection resistors are supplied to the microcomputer 11. The microcomputer 11 judges the rotation states of the fan motors in accordance with the binary signals as the rotation detection results and, rotation detection results from rotation sensors in the fan motors, and controls the supply of a power from a power supply circuit 1. Therefore, the rotation of the fan motor can be detected securely, malfunctioning can be avoided and the reliability can be improved.