Special pattern in cosmic microwave background (CMB) polarization, B-modes, is a smoking-gun signature of primordial gravitational waves. Q/U Imaging ExperimenT-II (QUIET-II) is being proposed to detect the B-modes using the world's largest coherent polarimeter array (2,000 channels). Having a large number of polarimeters is the generic strategy of the CMB polarization experiments to improve the statistics. Unique detection technique using QUIET's polarimeters, modulation/demodulation scheme, enables us to directly extract polarization signal. The extracted signal is free from non-polarized components and intrinsic 1/f noise. We developed a data readout system including on-board demodulation function for the QUIET-II experiment. We employ a "master" clock strategy. This strategy guarantees phase matching between the modulation on the polarimeters and the demodulation on ADC modules. The single master organizes to generate all carrier clocks and distribute them to each module. The developed electronics, clock modules and the ADC modules, fulfill requirements. Tests with similar setup to the real observation proved that the system works properly. Their performances are confirmed to be suitable for the B-mode measurements.