A semiconductor optical amplifier includes a conductive region that is provided on a substrate and allows light transmission, and a nonconductive region that is provided around the conductive region and prohibits light transmission. The conductive region includes a first region including a light-coupling portion to which light from an external light-source unit is coupled, and a second region having a narrower width than the first region and connected to the first region through a connecting portion, the second region including a light-amplifying portion amplifying the light from the light-coupling portion by propagating the light in a predetermined propagating direction along a surface of the substrate, the light-amplifying portion outputting the amplified light in a direction intersecting the surface of the substrate. Seen in a direction perpendicular to the surface of the substrate, the semiconductor optical amplifier includes a portion where a width of the conductive region is continuously reduced from the first region to the second region.