A rotor of a sensor-less motor has a multiplicity of magnetic poles, with one magnetic pole giving a smaller magnetic flux than other the magnetic poles. A coil is provided at a predetermined position external to the rotor. An induction signal is generated in the coil as the magnetic flux through it is changed by the rotation of the rotor. An index signal indicative of the reference rotational position of the rotor is generated based on the change in the induction signal. Thus, the reference rotational position of the disk of an information recording-reproducing unit, for example, can be detected without utilizing any Hall elements.