A semiconductor laser bar 2 is mounted onto a liquid-cooled heat sink 1. A molybdenum reinforcement member 3 is fixed onto the surface opposite to the surface on which the semiconductor laser module 2 is mounted. The molybdenum has a linear expansion coefficient less than that of the heat sink 1. Sub-mounts are preferably made of a Cu—W alloy, more preferably of the reinforcement member 3 molybdenum. In this case, the stresses that are imposed on the heat sink 1 when being expanded or contracted can cancel each other out.