PURPOSE: To detect the velocity of rotation of a three-phase brushless synchronous motor on the basis of an induced voltage in each phase during an electricity-feeding suspension period.

CONSTITUTION: A three-phase brushless synchronous motor is constituted in such a way that square waves, at a prescribed electricity-feeding angle, which are generated by a three-phase driving device 2 are supplied to a three-phase brushless synchronous motor 1. The three-phase brushless synchronous motor is provided with NOR circuits 16, 17, 18 which generate electricity-feeding suspension pulses corresponding to the electricity-feeding suspension period of each phase and with differentiation circuits 6, 7, 8 which shift the phase of an induced voltage induced in each phase of the three-phase brushless synchronous motor 1. In addition, the synchronous motor is provided with analog switches 9, 10, 11 which pass outputs of the differentiation circuits 6, 7, 8 during an electricity- feeding suspension pulse, with an addition circuit 12 which composes voltage waveforms obtained by the analog switches 9, 10, 11 and with an absolute-value operation circuit 13.