Carriers are injected into a plurality of quantum dots by applying a bias voltage to a semiconductor region of a semiconductor optical amplifier, the plurality of quantum dots for three-dimensionally confining carriers being distributed in the semiconductor region. An optical pulse signal at a bit rate of 2 Gb/s or higher is input to the semiconductor optical amplifier which amplifies the input optical pulse signal by generating induced emission by optical transition of the carriers in the quantum dots. An optical signal processing method capable of high speed and stable operation is provided.