PURPOSE: To effect actual time processing accurately and economically, by counting time between slits of velocity detector and reading out a velocity data table with this counted value as an address.

CONSTITUTION: A circular disc 2 provided with 2 pairs of slit groups of different phases set with the specified angle on concentric circles is installed fixedly on a motor shaft and these slit edges rotating basing upon a transmitted light beam is detected by 2 pairs of detecting means 30. Time between these neighboring edges of the 2 pairs of slit groups is counted by a between-edges counting means 31. Then, velocity data corresponding to the rotating time accommodated in a memory means is read out by a means 32 which reads out velocity data with the counted value as an address. By this arrangement, the between-slits time can be detected in terms of the actual time and with an accurate velocity detection without complicated arithmetic operations.