A digital filter is configured to convert, into a digital value, the duty ratio of a control signal subjected to pulse width modulation according to a target toque to be set for a fan motor to be driven. A sampling circuit is configured to perform sampling of the output value of the digital filter at a sampling timing that is asynchronous with respect to the cycle of the control signal, so as to generate a torque instruction value. A driving circuit is configured to drive the fan motor according to the torque instruction value thus generated.