PURPOSE: To form the highly accurate resolver signal in a simple, compact constitution, by using light sources which are turned ON and OFF by sine and cosine reference signals, and generating an incremental shaft encoder signal.

CONSTITUTION: The incremental encoder 1 is formed by a rotary grating plate 1a, fixed grating plates 1b and 1c, light sources LED 2a and 2b whose ON and OFF are controlled by a pair of the reference cosine and sine signals cosωt and sinωt, respectively, a pair of light receiving elements 3a and 3b, and 3c and 3d corresponding to the LED 2a and 3b, respectively, and the like. The outputs of the encoder 1 are processed in differential amplifirs Q1a and Q1b, operation amplifiers Q2 and Q3, and the like, and the resolver signal sin(ωt+ θ) is outputted, where θ is a rotary angle of an encoder shaft. In this plain, simple, compact constitution of an electronic circuit, since the optical signals are turned ON and OFF and spikes of an electric signal are not generated, highly accurately resolver signals can be generated.