A method for manufacturing a wavelength division multiplexing transmission apparatus, includes arranging laser devices in a line and fixing the laser devices to a first substrate, fixing to a second substrate reflectors arranged to multiplex laser beams emitted from the laser devices and to generate wavelength multiplexed light, and arranging collimator lenses on paths of the laser beams, adjusting positions of the collimator lenses so that the laser beams making up the wavelength multiplexed light become parallel and, thereafter, fixing the collimator lenses to the first substrate.