In a semiconductor optical device, a first conductive type semiconductor region includes first and second semiconductor portions. The first and second semiconductor portions are made of nitride mixed semiconductor crystal. This first semiconductor portion has a first region and a second region. The second semiconductor portion is provided on the first region of the first semiconductor portion. A second conductive type semiconductor region is made of nitride mixed semiconductor crystal. The second conductive type semiconductor region includes a first region and a second region. This second region of the first semiconductor portion of the first conductive type semiconductor region and the second region of the second conductive type semiconductor region constitute a pn junction. The sides of the second semiconductor portion of the first conductive type semiconductor region and the second region of the second conductive type semiconductor region constitute a pn junction. An active layer is made of nitride mixed semiconductor crystal the active layer is provided between the second semiconductor portion of the first conductive type semiconductor region and the first region of the second conductive type semiconductor region. The bandgap energies of the first conductive type semiconductor region and the second conductive type semiconductor region are greater than that of the active layer.