A small and thin surface-mount type optical semiconductor device having high air tightness, which can be manufactured at a reduced cost includes: a base 2 formed of a glass substrate; a recess 5 formed on a first main surface 3 of the base; a through hole 7 extending from a bottom portion 4 of the recess to a second main surface 6 of the base; an inner wall conductive film formed on an inner wall surface of the through hole; a wiring pattern 9 made of a conductive film formed around an opening of the through hole on the bottom portion of the recess so as to be connected electrically to the inner wall conductive film; an optical semiconductor element 8 bonded to the wiring pattern via a conductive bonding material 14; a terminal portion 10 made of a conductive film formed around an opening of the through hole on the second main surface such that it is connected electrically to the inner wall conductive film; and a metal portion 13 bonded to the inner wall conductive film to clog the through hole.