The present invention relates to a semiconductor optical amplifier, the semiconductor optical amplifier including: a plurality of optical amplification regions arranged in series; a passive waveguide region provided between optical amplification regions; and first and second electrodes provided on an upper surface of each of the optical amplification regions. The passive waveguide region electrically insulates between the first electrodes and between the second electrodes of the adjacent optical amplification regions and optically connects the adjacent optical amplification regions. The semiconductor optical amplifier electrically connects the first electrode and the second electrode of the respective adjacent optical amplification regions so that the plurality of optical amplification regions are electrically connected in cascade, and feeds power to the optical amplification regions at both ends of arrangements of the plurality of optical amplification regions thereby driving the plurality of optical amplification regions.