An optical semiconductor device includes: an n-type semiconductor substrate (1); an n-type cladding layer (2) provided on the n-type semiconductor substrate (1); an active layer (3) of a semiconductor laser provided on the n-type cladding layer (2); a waveguide layer (4) of a waveguide provided on the n-type cladding layer (2) and having a side facing a side of the active layer (3); a p-type cladding layer (5) provided on the active layer and the waveguide layer (4); and a middle layer provided between the side of the active layer (3) and the side of the waveguide layer (4), provided betweenthe n-type cladding layer (2)and the waveguide layer (4), not provided on the active layer (3), and having a band gap greater than a band gap of the waveguide layer (4).