A gallium nitride-based semiconductor optical device is provided that includes an indium-containing gallium nitride-based semiconductor layer that exhibit low piezoelectric effect and high crystal quality. The gallium nitride-based semiconductor optical device 11a includes a GaN support base 13, a GaN-based semiconductor region 15, and well layers 19. A primary surface 13a tilts from a surface orthogonal to a reference axis that extends in a direction from one crystal axis of the m-axis and the a-axis of GaN toward the other crystal axis. The tilt angle AOFF is 0.05 degree or more to less than 15 degrees. The angle AOFF is equal to the angle defined by a vector VM and a vector VN. The inclination of the primary surface is shown by a typical m-plane SM and m-axis vector VM. The GaN-based semiconductor region 15 is provided on the primary surface 13a. In the well layers 19 in an active layer 17, both the m-plane and the a-plane of the well layers 19 tilt from a normal axis AN of the primary surface 13a. The indium content of the well layers 19 is 0.1 or more.