PURPOSE: To provide the optical semiconductor module with which the need for the conventional optical axis adjustment operation is eliminated, man-hours for assembly and the number of parts are decreased and high productivity is possible.

CONSTITUTION: A part (stem 7a) of a stem 7 and a cap 3 are designed to have the same outside diameter size as the outside diameter size of a ferrule 10 in their outer peripheral parts (more specifically, these parts are so machined as to have the outside diameter smaller by about 1 to 5μm than the inside diameter of a ferrule insertion hole 2). This module consists of a structure to insert three members; the ferrule 10, the cap 3 and the stem 7a into the ferrule insertion hole 2. Then, the optical parts are positioned simply by the machining accuracy of the members and the need for the optical axis adjustment operation to be required heretofore is eliminated. The man-hours for assembly and the number of parts are correspondingly decreased and the high productivity is attained. The cost of production is thus reduced.