PURPOSE: To enable real-time detection of a scale at the machining speeds of all sorts by providing a data correcting circuit which uses a data table to correct data latched by a data latch circuit to one corresponding to reference speed.

CONSTITUTION: The timing at which the input and output ends of a mark pass is detected and a latch speed is detected according to the difference of time that both ends pass, and also the data of moving positions of one of the ends according to the timing of which the closed loop of a moving body is controlled is latched by a data latch circuit 24. A table indicating the relation between latch data and latch speed is stored by a data table, and data latched by the data latch circuit 24 is corrected using the data table to one corresponding to reference speed.