PURPOSE: To accurately read the quantity of light by collimating diffused light beams from a convergence spot into parallel light by a lens for the parallel light, converting the light beams by a condenser lens and splitting into two beams by a reflecting mirrors which has a sharp edge, and converting the light beams photoelectrically by a photodetector.

CONSTITUTION: The light beams which are converged on an original 2 and diffused from the convergence spot 1 are returned by a return mirror 3, the optical axis of the diffused light beams is set in a moving direction, and the diffused light beams are made parallel by the collimator lens 4. This parallel beams are image-formed by the condenser lens 5 and split into two beams in a positive and a negative opposite direction by a wedgelike reflecting mirror 6 installed nearby the image formation position, and the quantities of the separated beams are read by the photodetector 7. Therefore, even when the return mirror 3 and collimator lens 4 are moved in the optical-axis direction, the spot obtained by the condenser lens 5 has no shift in position and the quantities of light are prevented from varying with the diameter of the converged spot. Consequently, the accurate quantities of light are readable.