In an optical sensor, a light emission system emits an irradiated light of a linear polarization in a first polarization direction toward a surface of a target object having a sheet shape from an incident direction which is inclined with respect to a normal direction of the surface. A first light detection system includes a first light detector arranged on a first light path of a specular reflected light, which is emitted from the light emission system and is specularly reflected from the target object. A second light detection system includes a second light detector arranged on a second light path of a diffuse reflected light which is diffusely reflected from an incident plane on the target object. The second light detector receives second light passed by an optical element which passes a linear polarization component of a second polarization direction perpendicular to the first polarization direction.