An optical waveform reshaping device, including a semiconductor optical waveguide which has an active layer, wherein: optical amplification regions and optical absorption regions are installed alternately along the semiconductor optical waveguide; one optical amplification region is set longer than the other optical amplification regions so that a desired amplification factor can be obtained when power of an input optical signal is at an ON level; a power level is maintained by the other optical amplification regions excluding the one optical amplification region and by the optical absorption regions when the power of the input optical signal is at the ON level; and when the power of the input optical signal is at an OFF level, the input optical signal is absorbed by the optical absorption regions so that a power level of an output optical signal will not be higher than the power level of the input optical signal.