PURPOSE: To measure easily the density and color of printed matter, etc. with high resolving power by unitizing, separating and constituting independently optical sensor heads of an integral light projection and detection type by each of light emitting units for three primary colors.

CONSTITUTION: The optical sensor heads 1W3 are made into the construction in which 6 pieces of light projecting elements (optical fibers for transmitting the light from a light source for RCB) 1RTW6RT are provided around the central photodetector RJ and plural pieces of such min constituting elements are arranged regularly with good accuracy. The projecting light corresponding to R is uniformly projected from the elements 1RTW6RT when the head 1 is disposed in the position proximity to a surface 5 to be detected. The light is evenly irradiated to the surface 5 (the part of areas A1, A2) by utilizing the wide angle characteristic of the optical fibers, etc. and the light of the area A3 is consequently condensed to the photodetector RJ. The same operation is also applied to the optical sensor heads 2, 3 for G, B as well, by which the density of the surface 5 is measured by each color with high resolving power.