In the drive device for a three-phase synchronous motor, if one of the two rotational position detectors is defective, it is necessary to identify the rotational position detector having abnormal output and is in the detective state. To do this, three or more rotational position detectors are required, which increases costs of the rotational position detectors. By comparing the outputs of two rotational position detectors, an abnormality of ether one of the two detectors is detected. The abnormal detector is identified with the use of rotational position estimating means. The drive of the three-phase synchronous motor is controlled with the use of the output θ of the properly functioning one of the first and second rotational position detectors.