PURPOSE: To simplify the positioning of component elements while obtaining two sets of pulse trains in a desired phase difference by setting a phase difference for light quantity detection pulse trains of a light receiving element with axial displacement of a rotary plate and a fixed slit plate relatively.

CONSTITUTION: As a rotary plate 1 is moved in the direction B while the light source 20 is shifted in the direction A along the optical axis PC, the quantity of light received by a light receiving element 31 advances in the phase while that by a light receiving element 32 lags. When the rotary plate 1 is shifted toward slits 11 and 12 of a stationary plate 10, the phase of the quantity of light received by the light receiving element 31 lags while that by the light receiving element 32 advances. When the stationary slit plate 10 is shifted in the direction A while the rotary plate 1 is fixed with respect to the direction of the optical axis PC, a phase deviation occurs opposite to that caused when the rotary plate 1 is shifted. Such a shift in the phase according to the displacement of the slit plate in the direction of the optical axis is used to set a phase difference for the quantity of light received by the light receiving elements 31 and 32.