To obtain an optical semiconductor device which allows easy production and has versatility.

A substrate 40 mounted with an optical element 20, a coupling means 80 for light transmittably coupling this optical element 20 and an external optical waveguide and a semiconductor chip 30 for driving the optical element is housed in a package 90 in the state where its side edge is fitted and detained into a recessed part 94 on the inner side of the side wall of a package 90. The coupling means 80 is inserted into a window 100 of the side wall of the package 90 to expose the external optical waveguide juncture 82 of the coupling means 80 to the outer side of the window 100. The electrodes of the optical element 20 and the semiconductor chip 30 are electrically connected to the inner end of leads 120 for connecting the external electronic circuits disposed on the side wall of the package 90. The apertures 92, 96 of the package 70 are closed by a cap 130. The optical element 20 and the semiconductor chip 30 for driving the optical element form the built-in optical semiconductor device.