PURPOSE: To stabilize an origin signal by providing origin signal forming light projecting and receiving elements, where light is interrupted with plural origin slits arranged at unequal intervals on a phase plate and a code wheel in respective circumferential directions, and a waveform shaping means.

CONSTITUTION: Slits 4 and 6 are provided at equal intervals in respective circumferential parts of a phase plate 3 and a code wheel 5 attached to a housing (omitted in a figure) and a detection shaft 7, and the light from a light projecting element 1 is transmitted through slits 4 and 6 and is received by a photodetector 2. Origin slits 10 and 11 are arranged at unequal intervals in respective circumferential parts of the plate 3 and the wheel 5, and light projecting and receiving elements 12 and 13 are arranged on both sides of slits 10 and 11, and the element 13 is connected to the waveform shaping means (omitted in the figure). Thus, the peak value of the output of the element 13 is made larger considerably than the second peak value, and its rise and fall are made sharp.