An optical element and an optical transmission member are fixedly secured onto a substrate so as to be coupled to each other. In this substrate, a through hole is formed between an optical element secured portion and an optical transmission member secured portion. In an attempt to assemble these optical transmission member and optical element so as to make the coupling at an optimal position, first, the position of the optical transmission member is adjusted to an optimal position, and after measuring the position by a laser micrometer, the optical transmission member is fixedly secured thereto by using a soldering material. Thus, measurements are again carried out by the laser micrometer to detect an amount of deviation from the measured value before the securing process, and the secured portion is again fused so that the optical transmission member is shifted based on the amount of deviation, and again fixedly secured thereon.