PURPOSE: To make a fine adjustment of the quantity of light incident to a photodetecting element easily and continuously and to eliminate the influence of external light by providing a light shielding means for adjusting the quantity of transmitted light to a light guide means which guides light from a light emitting element to a photodetecting element.

CONSTITUTION: The cylindrical guide member 6 which has specific length for giving directivity to incident light that the photodetecting element 5 photodetects is fitted to the front surface of the photodetecting element 5. A screw 7 is inserted into a screw hole at a specific position of the light guide member 6. When the screw 7 is turned and inserted into the light guide member 6, light incident from the light emitting element to the light guide member 6 is cut off patially by the insertion part of the screw 7 to reach the photodetecting element 5. Consequently, the extent of the insertion of the screw 7 is varied by turning the screw 7 to make a fine adjustment of the quantity of incident light to the photodetecting element 5 continuously.