A semiconductor IQ optical modulator in which a phase modulation unit is configured by a differential capacitively loaded traveling-wave electrode structure based on an SS line configuration, phase modulation units of adjacent channels are spaced apart from each other by 400 µm or more, a distance between main signal lines of the capacitance loading type structure is 60 µm or less, a DC phase adjustment electrode and a PAD are provided between an I side phase modulation unit and a Q side phase modulation unit, the DC phase adjustment electrode is spaced apart by at least 80 µm or more from a signal line of the phase adjustment unit, and a crosstalk characteristic between the adjacent channels is -30 dB or less in a required frequency band.