A closed-cycle gas refrigeration system, comprising: a compressor and an expander coupled to a single output shaft of a driving device; a cooler cooling the refrigerant gas on an outlet side of the compressor; a brine cooler cooling brine with the refrigerant gas made of air or nitrogen gas and brine cooling an object to be cooled; a cold energy recovery heat exchanger further cooling the refrigerant gas cooled by the cooler with the refrigerant gas returning from the brine cooler; and a sealed refrigerant gas supply/discharge system formed by an expansion tank hermetically containing refrigerant gas having a pressure of at least an atmospheric pressure and a gauge pressure of lower than 0.2 MPa, a compressor inlet-side connecting passage connecting the expansion tank and a compressor inlet-side refrigerant gas passage, and a first open/close valve provided in the compressor inlet-side connecting passage, characterized in that the system further comprises: a compressor outlet-side connecting passage connecting the expansion tank and a compressor outlet-side refrigerant gas passage; a pressure sensor detecting a pressure of refrigerant gas flowing in the compressor inlet-side refrigerant gas passage; a second open/close valve provided in the compressor outlet-side connecting passage; and a controller receiving a detection signal from the pressure sensor and thereby controlling the first open/close valve and the second open/close valve, such that the refrigerant gas pressure in the compressor inlet-side refrigerant gas passager is maintained within a preset range, wherein refrigerant gas is supplied from the expansion tank to the compressor inlet-side refrigerant gas passage via the compressor inlet-side connecting passage, and discharged from the compressor outlet-side refrigerant gas passage to the expansion tank via the compressor outlet-side connecting passage while the pressure sensor detects the refrigerant gas pressure in the compressor inlet-side refrigerant gas passage, such that the refrigerant gas pressure in the compressor inlet-side refrigerant gas passage is maintained within the preset range. a compressor and an expander coupled to a single output shaft of a driving device; a cooler cooling the refrigerant gas on an outlet side of the compressor; a brine cooler cooling brine with the refrigerant gas made of air or nitrogen gas and brine cooling an object to be cooled; a cold energy recovery heat exchanger further cooling the refrigerant gas cooled by the cooler with the refrigerant gas returning from the brine cooler; and a sealed refrigerant gas supply/discharge system formed by an expansion tank hermetically containing refrigerant gas having a pressure of at least an atmospheric pressure and a gauge pressure of lower than 0.2 MPa, a compressor inlet-side connecting passage connecting the expansion tank and a compressor inlet-side refrigerant gas passage, and a first open/close valve provided in the compressor inlet-side connecting passage, characterized in that the system further comprises: a compressor outlet-side connecting passage connecting the expansion tank and a compressor outlet-side refrigerant gas passage; a pressure sensor detecting a pressure of refrigerant gas flowing in the compressor inlet-side refrigerant gas passage; a second open/close valve provided in the compressor outlet-side connecting passage; and a controller receiving a detection signal from the pressure sensor and thereby controlling the first open/close valve and the second open/close valve, such that the refrigerant gas pressure in the compressor inlet-side refrigerant gas passager is maintained within a preset range, wherein refrigerant gas is supplied from the expansion tank to the compressor inlet-side refrigerant gas passage via the compressor inlet-side connecting passage, and discharged from the compressor outlet-side refrigerant gas passage to the expansion tank via the compressor outlet-side connecting passage while the pressure sensor detects the refrigerant gas pressure in the compressor inlet-side refrigerant gas passage, such that the refrigerant gas pressure in the compressor inlet-side refrigerant gas passage is maintained within the preset range.