PURPOSE: To shorten the delay of a detected speed for an actual speed even in case of the transient state where the speed is changed greatly, by correcting the calculated value of a speed pulse train having a frequency, which is proportional to the speed of a system to be detected, with the counted value of a reference counter.

CONSTITUTION: A presettable counter 3 which counts the speed pulse train generated in proportion to the speed of the system to be detected, the reference counter which counts reference clocks 5 having a certain frequency during the time from start of counting of the counter 3 to counting-up, and a means which obtains the preset value of the counter 3 on a basis of speed data Vi in accordance with the counted value of the reference counter and presets this value to the counter 3 are provided. The speed is detected by a means which calculates correcting data on a basis of speed data Vi and preceding calculated speed data.