To provide an optical semiconductor device adaptable to surface mounting which suppresses the mounting area min. and can be manufactured at a low cost, without lowering reliability of the device or increasing the number of manufacturing process steps.

Either of an upper mold 7 or lower mold 8 divided at the boundary of a lead 2a mounting an optical semiconductor chip is set larger than the other resulting exposing of the lead 2a, and the lead 2a is not drawn to the outside from a package. This enables the surface mounting without using an IC substrate, etc., there is no need to draw mounting leads to outside from the package surround as in the conventional Z forming work, so that the mounting area can be reduced.