A light distribution control device including a substantially plate-like or film-like light diffusion structure which transmits or reflects light and at least one surface of which has a number of ridges arranged parallel to each other. The section of each ridge taken in a direction perpendicular to the longitudinal direction substantially constitutes a part of a circle, the surface of each ridge being smooth enough not to cause significant irregular diffusion of light. The ratio of diffraction light quantity to incidence light quantity is adjusted through selection of the angle of circumference and maximum diameter of the section of each ridge. The projection of diffusion distribution onto a plane perpendicular to both the longitudinal direction of the ridges and the surface of the structure is substantially symmetrical with respect to the normal at the point of incidence independently of the angle of incidence of the light.