A refrigerant charging system includes: a reliquefaction system reliquefying boil-off gas generated in a liquefied gas storage tank by compressing the boil-off gas and subjecting the compressed boil-off gas to heat exchange with refrigerant supplied to a heat exchanger while circulating along a refrigerant circulation line; a buffer tank storing utility N2 to be supplied to the ship; a booster compressor receiving the utility N2 from the buffer tank, compressing the received N2, and supplying the compressed N2 to the refrigerant circulation line; and a first load-up line along which the N2 is supplied from the buffer tank to the refrigerant circulation line without passing through the booster compressor. Upon initial charging in a non-operation state of the reliquefaction system, the refrigerant circulation line is charged with refrigerant by supplying the N2 from by a pressure differential between the refrigerant circulation line and the buffer tank.