PURPOSE: To omit arithmetic processing by providing a photosensitive plate which has an amorphous silicon layer and electrodes along the light projection surface of a prism provided on an optical transmission line, and discriminating their colors from variation in the resistance value between the electrodes due to difference in wavelength between colors.

CONSTITUTION: The color sensor consists of plural photosensitive bodies each consisting of a light emission source 25, a prism 24 provided on the optical transmission line, an amorphous silicon layer 23 provided along the projection surface of the prism 24, and a couple of electrodes 22. Light emitted by the light emission source 25 is refracted on the contact surface between the prism 24 and the outside for the 1st time and passed through the prism, and then refracted on the contact surface with the outside for the 2nd time. Then, the angle 26 to the perpendicular at the time of the 2nd refraction varies with the wavelength of a color and the amorphous silicon layer 23 is irradiated with the light through an electrode corresponding to the color, so the resistance value between the electrodes is different for each light beam, thereby deciding the color without arithmetic processing.