PURPOSE: To improve the S/N in case of detecting an original, by inclining and disposing optical axes of a light emitting element and a light receiving element against a normal of the surface of a transparent plate.

CONSTITUTION: A light beam R1 from a light emitting element 3 irradiates an original 1 through glass 2, and a part of a light beam R2 which has been reflected irregularly by the surface of the original 1 is made incident on a light receiving element 3b through the glass 2 again. In this case, the light beam R1 from the element 3a is reflected by the incident surface of the glass 2, as well, but as for a sensor 3, its optical axis is placed in a state that it has been inclined by a prescribed angle θagainst a normal of the surface of the glass 2, therefore, a reflected light R3 is reflected in the direction being different from the element 3b, and not made incident on the element 3b, therefore, it does not become a noise. Also, even when a light beam is emitted into the air from the glass 2, it is reflected by the inside surface of the glass 2, but this reflected light R4 is not made incident on the element 3b either. Accordingly, against a light quantity S of the reflected light R2 of the original 1 in the element 3b, it does not occur that a light quantity N of the reflected rays R3, R4 from the glass 2 increases, therefore, the S/N in case of detecting an original is improved.