PURPOSE: To obtain a 90° phase difference signal with less errors by disposing two slits of a phase plate so as to face each other to upper and lower slits which are juxtaposed in the circumferential direction of a cord wheel and are inclined differently with respect to the axial center.

CONSTITUTION: The upper slit 6a and the lower slit 6b which are different in inclination of the cord wheel have the continuous shape of a parallelogram. On the other hand, the shape of the slits 4a, 4b of the phase plate 3 is the parallelogram as well and said slits are provided in the positional relation in which the phase difference of the light signals transmitted therethrough is 90°. These slits are disposed with respect to the pattern center in such parallel positional relation that the slit 4a faces the slit 6a and the slit 4b faces the slit 6b, respectively. If the position is shifted in the direction (y) shown in the figure (a), the positions of the slits 6a, 6b shift by β° in the same direction but the shifting directions are the same as shown by an arrow and the 90° phase difference relation of the light signals transmitted therethrough is the same. The 90° phase difference relation does not change with the same reason even if the position is shifted in the direction (x) shown in the figure (b).