PURPOSE: To enable obtaining from a light reception part a stable output close to a pulse waveform having proper duty with a sharp rising.

CONSTITUTION: Between a light reception parts 11 and 12, a rotational disk 13 is provided. Between the above rotational disk 13 and the light reception part 11 and 12, a fixed part 17 being formed a second slit hole 18 having a narrower slit width W2 than the slit width W1 of the first silt hole 14 formed on the rotational disk 13 is provided. With the slit hole 18, the light intensity reaching from the light reception part 11 to 12 is regulated so that constant light intensity is always given to the light reception part 12. By this, the output of the light reception part rises sharply to be a pulse shape waveform.