An edge-emitting semiconductor optical device comprises a first cladding layer, an active layer, and a second cladding layer. The first cladding layer is provided on a semiconductor substrate. The active layer is provided on the first cladding layer. The semiconductor substrate has a higher band gap than that of the active layer. The first cladding layer includes a first light-absorbing layer and a first light-transmitting layer. The first light-absorbing layer has a lower band gap than that of the active layer, and the first light-transmitting layer has a higher band gap than that of the active layer. The second cladding layer is provided on the active layer.