To detect the phase of a rotor in a synchronous motor or the like with high accuracy and at minimum calculation load.

The phase of the rotor is computed, based on the latest phase detection position and time value date, from the time measurements of two or more phase signals which are generated periodically at each prescribed phase interval with the rotation of the rotor and rotation angular velocity computed from the measurements. The computation of the rotation angular velocity is performed each time the phase signal occurs by the number given by a parameter value (m). The higher rotational speed which shortens phase signal generating time interval becomes, the more the operation frequency of the rotation angular velocity is reduced relatively, so that it is possible to reduce operation load, while ensuring high detection accuracy.