PURPOSE: To obtain accuracy of measurement independently of a speed by dividing the frequency of a rotation speed pulse signal by means of a presettable counter and by setting a counter-preset value for the following measurement of the speed according to an interval of pulse generation thus obtained.

CONSTITUTION: The rotation of a rotating body 1 is detected as a pulse signal by a pulse generator 2 and the frequency thereof is divided by a preset counter 3 whose frequency division ratio is decided by an outside instruction signal Nps. A counter 6 counts signals of a reference pulse generator 4 only for a period selected by an output of the preset counter 3, and a speed of rotation N is calculated by an arithmetical unit 7. Moreover, the arithmetical unit 7 leads out a preset value of the preset counter 3 for the measurement to follow.