An optical semiconductor device (1) has a semiconductor substrate (2) made of InP, an active layer (7) which is formed in parallel with a top surface (2a) of the semiconductor substrate (2) above the semiconductor substrate (2), an n-type first cladding layer (6) made of InGaAsP which is formed under the active layer (7), a p-type second cladding layer (8) made of InP which is formed under the active layer (7), and window regions (4a, 4b) which are formed at least one light-emitting facet of both light-emitting facets of the active layer (7). The window regions are formed between device facets (1a, 1b) from the light-emitting facet. A relationship is established in which, given that a refractive index of the n-type first cladding layer (6) is na, and a refractive index of the p-type second cladding layer (8) is nb, na>nb is obtained that the refractive index na of the n-type first cladding layer (6) is higher than the refractive index nb of the p-type second cladding layer (8), so as to deflect a distribution of electric field strength of a light generated at the active layer (7) toward the n-type first cladding layer (6) side.