In an optical semiconductor package according to the present invention, the component bearing surface of the stem having an outline made up of opposed first and second circular-arc parts, a first straight-line part which connects each end of the first and second circular-arc parts and a second straight-line part which connects each other end of the first and second circular-arc parts. The first and second circular-arc parts have a common center, and the first circular-arc part has a larger radius than that of the second circular-arc part. Thus, the larger space, the space between the first circular-arc part and the center, can be used for disposing the light detector or a polarization split element, and the unused space, the space between the second circular-arc part and the center, will be minimum, thereby realizing downsizing and cost reduction of the device.