One object of the present invention is to provide a natural gas liquefaction device which uses noncombustible gas as a refrigerant, and can reduce the power consumption a range of relatively low refrigerant pressure, and the present invention provides a natural gas liquefaction device including a compressor which is configured to compress a refrigerant containing noncombustible gas by a plurality of compression stages; a heat exchanger which is configured to cool and liquefy a natural gas to be a liquefied natural gas; a natural gas liquefaction line which is configured to introduce the natural gas into the heat exchanger and supply the liquefied natural gas to an outside; a first refrigerant line which is configured to introduce a refrigerant-1 passed through the compressor into the heat exchanger, and then further introduce the refrigerant-1 into a decompressor; a second refrigerant line which is configured to introduce the refrigerant-2 decompressed by the decompressor into the heat exchanger, and further introduce the refrigerant-2 into any one of a second compression stage and subsequent stages of the compressor; a third refrigerant line which is configured to be branched from the first refrigerant line and introduce at least a part of the refrigerant-1 into an expansion turbine; and a fourth refrigerant line which is configured to introduce the refrigerant-3 expanded by the expansion turbine into the heat exchanger, and further introduce the refrigerant-3 into a first compression stage of the plurality of compression stages provided in the compressor.