PURPOSE: To obtain an apparatus which enables the detection of a speed signal with limited ripples along with a better response by performing a constant current integration and a specified computation corresponding to the outputs of a rotary encoder.

CONSTITUTION: Outputs A0 and B0 of a 2-phase type rotary encoder are applied to an exclusive OR circuit 7, the output C0 is fed to a logic circuit 15 while the output C0 inverted 8 to a logic circuit 16 to obtain drive signals of analog switches S1WS8. Integration circuit 2 and 3 perform a constant current integration at intervals of 1/4 cycle of the outputs A0 and B0 with the action of analog switches, output a DC voltage whose maximum is proportional to the cycle and the maximum thereof is sampled held 4 at intervals of 1/4 cycle of the outputs A0 and B0. Based on the outputs A0 and B0, a discriminator circuit 5 feeds a constant voltage of a code varied with the direction of rotation to be divided 6 by the sample held output. As a result, a DC voltage can be detected which varies in the polarity with the direction of rotation while it is proportional to the rotational speed.