To make an optical semiconductor device have high integration and multifunction, to efficiently transmit a high-frequency signal, to enable an optical semiconductor element to operate normally and stably for long time by improving the hermetical reliability of the inside.

The package for housing an optical semiconductor element is provided with a metallic base 1 on which a rectangular through hole 1a piercing between upper and lower main surfaces is formed and arcuately expanded portions 1b are formed on four corners of the through hole 1a are formed; an input/output terminal 5 which consists of a planar portion 5b having a line conductor 5c formed thereon and provided with a placing portion of an optical semiconductor 2, and an upright wall 5a joined to one main surface of the planar portion 5b with one part of the line conductor 5c sandwiched and is fitted to the through hole 1a; and a metallic cover 3 in which a translucent member 4 is joined around an opening of the through hole 3b, and which is joined to the external peripheral portion of the upper main surface of the base 1.