We develop a cryogenic readout integrated circuit (ROIC) for a far-infrared astronomical image sensor. For far-infrared astronomical observations, detectors must be cooled down under liquid helium (LHe) temperature to reduce thermal dark noise. Then, ROICs also should be operated at the temperature. At cryogenic temperature, conventional bulk-MOSFETs, in particular NMOS FETs, show degradation such as kink effect and hysteresis in drain current. Thus, the ROICs fabricated by conventional bulk-CMOS process show poor performance at cryogenic temperature. In contrast, MOSFETs fabricated by a fully depleted silicon-on-insulator (FD-SOI) CMOS process show stable characteristics at cryogenic temperature. For the far-infrared image sensor, we designed a capacitive trans-impedance amplifier (CTIA). In this presentation, the first results of the CTIA demonstrated at LHe temperature will be reported.