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