PURPOSE: To secure the detection of an accurate velocity of the motor, by giving a comparison to the amount of generation of pulses delivered from the pulse generator connected to the motor and the pulse oscillator having a certain frequency each.

CONSTITUTION: Pulse train fp generated from pulse generator 5 connected to motor 2 is read into memory device 7; and at the same time pulse train fc given from pulse oscillator 19 having a certain frequency for oscillation is read into memory device 12 respectively. Then the values of devices 7 and 12 are transferred to memory devices 8 and 13 each. After this, pulse trains fp+α and fc+β delivered from generator 5 and oscillator 19 are stored in devices 7 and 12 each. At the same time, the values of devices 8 and 13 are subtracted from the values of devices 7 and 12 via subtractors 9 and 14, and subtracted values α and β are stored in memory devices 10 and 16 respectively. Then value α of device 10 is divided by value β of device 15 through devider 16, and the value of memory device 18 decided by the system is multiplied by result γ through multiplier 17. Thus actual velocity V is obtained.