By using the power generated by an expander by an expansion of material gas, the outlet pressure of a compressor is increased, and a requirement on the cooling capacity of a cooler is reduced. The liquefaction system (1) for natural gas comprises a first expander (3) for generating power by expanding natural gas under pressure as material gas; a first cooling unit (11, 12) for cooling the material gas depressurized by expansion in the first expander; a distillation unit (15) for reducing or eliminating a heavy component in the material gas by distilling the material gas cooled by the first cooling unit; a first compressor (4) for compressing the material gas from which the heavy component was reduced or eliminated by the distillation unit by using the power generated in the first expander; a second heat exchanger for exchanging heat between the material gas introduced into the first compressor and the material gas compressed by the first compressor; and a liquefaction unit (21) for liquefying the material gas compressed by the first compressor by exchanging heat with a refrigerant.

The liquefaction system comprises a high-temperature refrigerant circuit and a low-temperature refrigerant circuit. The system further comprises a first high-temperature heat exchanger, wherein the feed gas is in heat exchange with a first stream of vaporizing first refrigerant and is cooled thereby, and a second high-temperature heat exchanger, wherein compressed first refrigerant of the first refrigerant circuit and compressed second refrigerant of the second refrigerant circuit are in heat exchange with a second stream of vaporizing first refrigerant and are cooled thereby. A low-temperature heat exchanger is further provided, wherein cooled feed gas from the first high-temperature heat exchanger and cooled second refrigerant from the second high-temperature heat exchanger are further cooled and liquefied in heat exchange with a flow of vaporizing second refrigerant.