Abstract: In a device for coupling optically a light source with an optical transmission path, the present invention interposes an optical transmission path structure at the center of the distance between them in order to improve the optical coupling efficiency. The optical transmission path structure consists of a plurality of light transmitting elements arranged so that the light from the light source is received by the structure as incident light and is sent out therefrom as outgoing light at an outgoing angle which is equal to the angle of incidence. The light transmitting elements are arranged in parallel with the optical path of the light emitted from the light source and according to this arrangement, the system of the present device becomes equivalent to a system in which the positions of the light source are located at points of .+-.l.sub.1 from the light outgoing surface of the optical transmission path structure.