An optical semiconductor module having a large endothermic amount of an electronic cooling element can be provided, even if the area of the bottom plate of a package is the same. A package 11 includes two or more units of electronic cooling element 16 mounted therein. Each unit of the electronic cooling element is inserted through the space between inner juts 14a of ceramic feedthrough of the package 11 and a bottom plate 13, and is fixed to the bottom plate. The plural units of electronic cooling element are connected in series by one or more copper piece. The total area of junction between the two or more units of electronic cooling element and the bottom plate area of the package 11 occupies 75% or more of the area of the bottom plate. Thus, the ratio of the area of junction between the bottom plate and the electronic cooling element as a whole to the area of the bottom plate of the package can be increased.