A semiconductor optical device which has a ridge structure includes a waveguide area between paired mesa trenches; first and second mount areas disposed outside the mesa trenches; a first spacer layer disposed in a first mount area and a second spacer layer disposed in a second mount area; a first metal layer electrically connected to an upper cladding layer in the waveguide area and extending from the waveguide area over the first mount area; and a second metal layer disposed over the second mount area. Thicknesses from a back surface of the semiconductor optical device to the first metal layer in the first mount area and to the second metal layer in the second mount area are both larger than thickness from the back surface to the first metal layer in the waveguide area.