PURPOSE: To provide a rotor position detecting circuit, capable of controlling a commutator less motor stably by the induced voltage of an armature even when the number of rotations is changed.

CONSTITUTION: This circuit is constituted of low-pass filter groups 6 corresponding to the 3-phase windings of an armature, operational amplifier groups 7 and comparing circuit groups 8. The terminal voltages Va1, Vb1, Vc1 of the armature of a motor pass through respective low-pass filters 6 and operational amplifiers 7a-7c, then, are compared with reference voltages outputted by voltage dividing circuits 11a, 11b, 11c in comparators 8. The output signals Va3, Vb3, Vc3 of the comparator 8 are inputted into a signal processing unit. The signal processing unit produces a PWM signal to control a driving circuit unit for driving an inverter. The integrating circuits of the operational amplifiers 7a-7c are constituted of a plurality of resistors, switching elements and capacitors respectively whereby the values of resistances are changed in corresponding to the numbers of rotations when the number of rotations of a motor is changed largely, a phase difference between an input signal and an output signal is approached to 90° and stabilized control can be possible.