According to the present invention, a fixing method by soldering is provided which attains, upon fixing to a substrate a cylindrical optical part which includes a lens, a photo-semiconductor device having a light emitting portion or a light receiving portion located substantially at a focus position of the lens, and terminals connected to the photo-semiconductor device, mechanical fixation of the cylindrical optical part to the substrate and electric connection of the terminals to the substrate at a time. According to another aspect of the present invention, a method of collectively fixing an optical part and an electric part on a substrate by soldering is provided. A heating block is divided into a portion made of a substance having a high coefficient of thermal conductivity and another portion made of another substance having a low coefficient of thermal conductivity. The portion made of the substance having a high coefficient of thermal conductivity is contacted with a reverse face of a portion of a substrate at which an optical part is mounted while the portion made of the substance having a low coefficient of thermal conductivity is contacted with the reverse face of another portion of the substrate at which an electric part is mounted.