To provide a means for avoiding the resonance state of a fan.

A pulse signal SP with reference duty ratio corresponding to a target speed is outputted to a chopper circuit (S1), and a DC voltage is applied to a fan motor. While the fan motor is being rotated, temperature in a machine room where the fan motor is provided is detected (S3), and the reference duty ratio is reset when the detected temperature is equal to reference one (S5). When the detected temperature is higher than the reference one, the deviation width of the speed is calculated based on the temperature-speed characteristics of the fan motor being given in advance (S7), and the duty ratio of the pulse signal PS is decreased according to the deviation width (S8). On the other hand, when the detected temperature is lower than the reference one, the duty ratio of the pulse signal SP is increased (S10), thus controlling the fan motor at a constant speed, and preventing resonance due to coincidence with the natural frequency of a blast blade in advance.