A semiconductor optical element comprises a semiconductor substrate, a waveguide, an active layer and a clad layer mounted on said substrate; said active layer comprising first region and second region coupled optically with each other, and having opposite signs of .alpha. parameters with each other, where said parameter is ratio of change of refractive index to change of gain following change of carrier density; a first electrode common to all regions attached on one side of said substrate; and second electrodes for each region attached on the other side of said substrate. The invention is used for an AM modulation laser, or an FM modulation laser.