The air flow rate of a fan driven by a motor controlled by an adjustable speed control equipment is automatically controlled by stopping the adjustable speed control equipment in response to a fault signal therefrom during operation with an inlet vane installed upstream of the fan in fully open state, simultaneously throttling the inlet vane at a preset speed to a degree of opening appropriate for the load by means of a control circuit, and, after a period for reduction of residual voltage of the motor, changing over to operation of the fan by a commercial power supply thereby to shift to control by the degree of opening of the inlet vane.