A semiconductor IQ modulator including at least two or more Mach-Zehnder, MZ, modulators configured with differential transmission lines in which two signal lines for transmitting radio frequency, RF, modulation signals comprising differential signals are coupled, wherein each of the differential transmission lines has an SS line configuration that one of the two signal lines for carrying a signal and the other one of the two signal lines for carrying a signal of opposite polarity constitute the differential transmission line, the SS line configuration consists of a straight lead-out line, a phase modulation unit, and a termination resistor (302a, 302b), the phase modulation unit includes the differential transmission lines having a differential capacitively loaded traveling-wave electrode, CL-TWE, structure as two phase modulation electrodes (804XIa, 804XIb, 804XQa, 804XQb), the two phase modulation electrodes (804XIb, 804XQa) of the phase modulation units between adjacent MZ modulators are spaced apart by at least 400 μm or more, a distance between main signal lines (804IXa, 804IXb) of the differential CL-TWE structures is 60 μm or less, a DC phase adjustment electrode (808XI, 808XIa, 808XIb) for adjusting an operating point of the MZ modulator and a PAD for applying voltage to the DC phase adjustment electrode are provided between the phase modulation unit on an I channel side and the phase modulation unit on a Q channel side, the DC phase adjustment electrode is spaced apart from the phase modulation electrode of the phase modulation unit by at least 80 μm or more, and near-end and far-end crosstalk characteristics of a differential signal between adjacent channels are -30 dB or less in a required frequency bandwidth. each of the differential transmission lines has an SS line configuration that one of the two signal lines for carrying a signal and the other one of the two signal lines for carrying a signal of opposite polarity constitute the differential transmission line, the SS line configuration consists of a straight lead-out line, a phase modulation unit, and a termination resistor (302a, 302b), the phase modulation unit includes the differential transmission lines having a differential capacitively loaded traveling-wave electrode, CL-TWE, structure as two phase modulation electrodes (804XIa, 804XIb, 804XQa, 804XQb), the two phase modulation electrodes (804XIb, 804XQa) of the phase modulation units between adjacent MZ modulators are spaced apart by at least 400 μm or more, a distance between main signal lines (804IXa, 804IXb) of the differential CL-TWE structures is 60 μm or less, a DC phase adjustment electrode (808XI, 808XIa, 808XIb) for adjusting an operating point of the MZ modulator and a PAD for applying voltage to the DC phase adjustment electrode are provided between the phase modulation unit on an I channel side and the phase modulation unit on a Q channel side, the DC phase adjustment electrode is spaced apart from the phase modulation electrode of the phase modulation unit by at least 80 μm or more, and near-end and far-end crosstalk characteristics of a differential signal between adjacent channels are -30 dB or less in a required frequency bandwidth.