An optical semiconductor module including a base having installed on an optical fiber and an optical semiconductor element, and a package which houses the base on a bottom thereof and has a first side wall with an optical section through which the optical fiber is led and a second side wall facing the first side wall, where the base is cut off to form a curved surface with respect to the bottom at a lower corner on a side of the base facing the second side wall of the housing, and a ratio of r/t is from 0.4 to 1.0, where t is a thickness of the base, and r is a curvature radius of the curved surface.