A polarization-independent SOA is provided which uses an InP substrate (11) as a semiconductor substrate and uses GaInNAs having introduced tensile strain as an active layer (14). With this configuration, the polarization independence is achieved by introducing the tensile strain, and high saturation optical output power is realized by reducing the film thickness of the active layer (14) as well as the gain peak wavelength is increased by reducing the band gap of the active layer (14) through use of GaInNAs made by adding nitrogen (N) to GaInAs as a material of the active layer (14) so as to achieve high gain especially in C-band and L-band even when band filling exits at the time of injecting a high current into the active layer (14).