PURPOSE: To prevent falling paper dust, applied material, and the like from being accumulated on an optical axis so as to prevent wrong detection by forming a recessed part or a hole part between a light emitting element and a light receiving element and providing the optical axis passing parts of the light emitting element and the light receiving element at the side wall faces of the recessed part or the hole part.

CONSTITUTION: A recessed part or a hole part 5 is formed between a light emitting element 1 and a light receiving element 2, as well as the optical axis passing parts of the light emitting element 1 and the light receiving element 2 are provided at the side wall faces of the recessed part or the hole part 5. Paper dust, applied material, and the like are thereby prevented from being accumulated on the side wall faces of the recessed part or the hole part 5. The light irradiated from the light emitting element 1 and injected into the light receiving element 2 passes these side wall faces and therefore is not influenced by the paper dust and the applied material.