Illuminating light emitted from a light source is led via a condenser lens, an optical fiber, a half-silvered mirror, and a condenser lens to a reflector. The reflector reflects the illuminating light to a CCD device through a telecentric optical system which comprises the condenser lens, the half-silvered mirror, and an aperture member. The illuminating light applied to the CCD device is greatly reduced in amount as it passes through a light-transmissive sheet-like body twice. The light-transmissive sheet-like body itself or an edge thereof can be detected with high accuracy even if the light-transmissive sheet-like body has a high transmittance.