A photodetector includes a light emitting portion for applying light, a light receiving portion for sensing the light, and a light guiding member 3 for guiding the light from the light emitting portion to a surface to be measured and guiding detection light from the surface to be measured to the light receiving portion. The light guiding member has a sheet-like optical transmission medium, which is disposed at a portion facing to the surface to be measured and transmits the light by internal reflection. The sheet-like optical transmission member has an optical aperture facing to the surface to be measured.