A semiconductor optical element includes a semiconductor layer portion that includes an optical waveguide layer. The semiconductor layer portion contains a first impurity having a function of suppressing atomic vacancy diffusion and a second impurity having a function of promoting atomic vacancy diffusion, between a topmost surface of the semiconductor layer portion and the optical waveguide layer. The semiconductor layer portion includes two or more regions that extend in a deposition direction with different contents of at least one of the impurities. At least one of the two or more regions contains both the first impurity and the second impurity. The two or more regions have different degrees of disordering in the optical waveguide layer achieved through atomic vacancy diffusion and different band gap energies of the optical waveguide layer.