PURPOSE: To prevent the control performance from being deteriorated due to an erroneous detection of a speed in a low speed area, when a speed of a motor is detected by a time intervals method, and a pulse difference method, in a low speed area and in a high speed area, respectively, in digital position control of the motor for using an output pulse of an encoder as a position detecting signal.

CONSTITUTION: In a step 1, a detection speed ωP(i) is calculated by a pulse difference method. In a step 2, this ωP(i) is compared with a speed ωO at which speed accuracy of a time interval method and the pulse difference method coincides with each other, and when ωP(i) is smaller than ωO, the operation advanced to a step 3, a detection speed ωT(i) is calculated by the time interval method, and in a step 4, whether this ωT(i) causes an erroneous detection due to a noise or not is decided. In such a state, in the case the speed ωT(i) by the time interval method does not satisfy an inequality shown in the step 4, it is decided that the speed detection is erroneous due to the cause of a high frequency noise, etc., and in a step 4-1, a speed value ω(i-l) in the previous sampling time is adopted as a speed ω(i) in the present sample.