PURPOSE: To detect accurately even a glossy printed face by providing an irregularly reflective transparent plate in front of light emitting elements and photodetectors.

CONSTITUTION: Plural photodetectors 18 and light emitting elements 21 are arranged as pairs, and a lens plate 13 and a transparent plate G are arranged in front of them. One of both faces of this transparent plate G are made irregularly reflective. The light emitted from light emitting element 21 is irradiated to the printed face through the lens plate 13 and the transparent plate G, and the reflected light is received by photodetectors 18 through the transparent plate G and the lens plate 13 and is converted to an electrical signal to detect the printed face.