NOVELTY - A nitrogen gas is compressed, cooled, pressurized and cooled. Cooled nitrogen gas is passed to booster, to obtain high-pressure nitrogen gas which is cooled, and then conveyed in primary heat exchanger (9) to exchange heat with natural gas and obtain low-pressure nitrogen gas in which portion is expanded and refrigerated in expansion machine. Then, obtained low-pressure nitrogen gas is introduced to primary heat exchanger in which natural gas is cooled, liquefied, over-cooled by low-pressure nitrogen gas. Nitrogen gas from heat exchanger is separated, to obtain liquid natural gas.    USE - Liquefaction of natural gas.    ADVANTAGE - The method liquefies natural gas with liquefying ratio of 100%, under high and low pressure conditions.    DETAILED DESCRIPTION - A nitrogen gas is compressed to 2-3 MPa, cooled to 30-45 degrees C and transferred to boosting end of low-pressure booster (2) to pressurize, and then cooled to 30-40 degrees C by secondary after-cooler (4). The cooled nitrogen gas is passed to boosting end of high-pressure booster (1) to obtain high-pressure nitrogen gas of 3.6-5.3 MPa. The high-pressure nitrogen gas is cooled to 30-45 degrees C by primary after-cooler (3), and then introduced in primary heat exchanger to exchange heat with natural gas and low-pressure nitrogen gas of 0.4-0.6 MPa is obtained from heat exchanger. The low-pressure nitrogen gas is cooled to -70 degrees C to -30 degrees C at the outlet of low-pressure expansion machine (6). Portion of low-pressure nitrogen gas is expanded and refrigerated in high-pressure expansion machine (5), and again expanded and refrigerated in low-pressure expansion machine, to obtain low-pressure nitrogen gas of 0.4-0.6 MPa. Then, obtained gas is introduced to primary heat exchanger in which natural gas is cooled, liquefied, over-cooled by low-pressure nitrogen gas. The nitrogen gas from heat exchanger is separated through throttling value (7), to obtain liquid natural gas which is passed to storage tank (8). The obtained liquid natural gas is recycled in primary heat exchanger.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of the process explaining liquefaction of natural gas. (Drawing includes non-English language text)    High-pressure booster (1)    Low-pressure booster (2)    Primary after-cooler (3)    Secondary after-cooler (4)    High-pressure expansion machine (5)    Low-pressure expansion machine (6)    Throttling valve (7)    Storage tank (8)    Primary heat exchanger (9)