A semiconductor optical device has a multilayer structure 30 including a first compound semiconductor layer 31, an active layer 33, and a second compound semiconductor layer 32. A second electrode 42 is formed on the second compound semiconductor layer 32 through a contact layer 34. The contact layer 34 has a thickness of four or less atomic layers. When an interface between the contact layer 34 and the second compound semiconductor layer 32 is an xy-plane, a lattice constant along an x-axis of crystals constituting an interface layer 32A which is a part of the second compound semiconductor layer in contact with the contact layer 34 is x2, a lattice constant along a z-axis is z2, a length along an x-axis in one unit of crystals constituting the contact layer 34 is xc′, and a length along the z-axis is zc′, (zc′/xc′)>(z2/x2) is satisfied.