PURPOSE: To increase the number of pulses without adding a detecting circuit group more, by splitting light from one projecting element through optical fibers and generating a pulse on the basis of a waveform composed by a photodetecting element after the light passes through a slit.

CONSTITUTION: Light from a projecting element 3 is branched to two optical paths by optical fibers 10AP and 10BP and then passed through a fixed slit plate 5 and a slit disk 2 to enter optical fibers 10AR and 10BR. When the pitch of a slit 2a is α°=360°/number-of-pulses (number of slits) and a gap between two slits 5a and 5b is expressed an an angle δ of opening from the center of the slit disk, δ=α×n-α/4. Pulses of light entering the optical fibers 10AR and 10BR are synthesized in a photodetecing element 4, whose output is supplied to a processing circuit 17 to output four fold pulses.