A semiconductor laser diode, in which an active layer is formed close to a bottom surface thereof, is fixed to a top surface of a substrate by means of solder layers in such a condition that the bottom surface of the semiconductor laser diode faces the top surface of the substrate, which is covered with a SiO₂ layer. The active layer is interposed between a pair of V grooves. The right and left solder layers partially fill a clearance formed between the bottom surface of the semiconductor laser diode and the top surface of the substrate, leaving a vacant space in the vicinity of the active layer and the V grooves. Accordingly, the distortion of and the residual stress exerted on the active layer can be decreed noticeably, so that the operation of an optical semiconductor module can be stabilized.