A lens assembly for transmitting an optical signal has a light-emitting element that diffuses the optical signal and a collimator lens that converts the optical signal diffused out of the light-emitting element from its diffused light into parallel light. The device also has a condenser lens that gathers the parallel light output from the collimator lens to focus the parallel light into an opening of an optical waveguide. The opening is provided in an end of the optical waveguide. An optical axis of the condenser lens is shifted toward the other 10 end of the waveguide by a predetermined distance with respect to an optical axis of the collimator lens.