An optical semiconductor module includes an optical semiconductor element; an optical fiber optically coupled with the semiconductor element; a base having an upper surface and a lower surface; and a package. The package has a bottom plate on which the base is mounted, a front wall having a hole through which the optical fiber is inserted, and a rear wall opposite to the front wall. The optical fiber and the optical semiconductor element are mounted on the upper surface. The base has a rear face formed on an end portion opposite to the front wall, and the rear face is positioned above the lower surface of the base.