Disclosed herein is a natural gas liquefaction process of pre-cooling natural gas using a closed loop pre-cooling cycle and liquefying the pre-cooled natural gas using a closed loop liquefying cycle, wherein the closed loop pre-cooling cycle includes first and second pre-cooling cycles in parallel for pre-cooling supplied natural gases together in the same first heat exchange region through the respective pure refrigerants, and the closed loop liquefying cycle includes at least one liquefying cycle for liquefying the pre-cooled natural gas through a mixed refrigerant, the first and second pre-cooling cycles being a closed circuit cooling cycle.