Between the lower stage in respective phases of inverter circuit and the negative potential side, shunt resistors are inserted for respective phases. The potential differences across shunt resistors occurring by current flowing through shunt resistors are amplified by amplifier unit. From the output of amplifier unit, current detecting unit detects current of each phase flowing through motor formed by a sensorless brushless DC motor. Rotation detecting unit calculates the rotation speed and position of motor based on the current detected by current detecting unit. This structure makes it possible to directly detect solely the current flowing through the windings of motor. Therefore, it becomes possible to accurately detect the current of the motor windings, and the ventilation device can maintain the ventilation air volume from a small air volume to a large air volume always at a constant value.