PURPOSE: To simplify circuit constitution and to detect a speed precisely in a short time by employing the constitution of a counter, register, and arithmetic circuit which is suitable for LSI implementation.

CONSTITUTION: A counted value T1 corresponding to the interval between a position pulse S1 and a reference pulse R1 is set in a B register 63. When a reference TP is generated, the counted value of a timer counter 62 is set in the B register 63. This counted value T2 corresponds to the interval between a position pulse S4 and a reference pulse R2. The arithmetic circuit 64 reads the counted value T2 out of the B register 63 and solves the 1st equation by using the preceding counted value T1 and a known period T0 to calculate an input necessary time T. Further, the arithmetic circuit 64 reads the number N of position pulses PP in the period between the reference pulses R1 and R2 out of an A register 61, and solves the 2nd equation to obtain speed information V.