Numerical aperture of transmitted light is varied to NAs by a transmitting lens. The numerical aperture, NAs, of the transmitted light is made larger than the numerical aperture, NAp, of an optical fiber, whereby the numerical aperture, NAf, of received light radiated from the optical fiber is made smaller as the transmission distance increases. A receiving optical system is arranged such that the receiving efficiency of the received light is increased by the provision of an aperture member according as the numerical aperture, NAf, is made smaller. This improves the receiving efficiency when the transmission distance is long and reduces a variation in the received light quantity even if the transmission distance is varied.