Disclosed is a system for reducing a heating value of natural gas. The system includes a heat exchanger to liquefy a portion of components having high heating values, a gas-liquid separator to separate the liquefied component, and a nitrogen adding mechanism to add nitrogen to remaining non-liquefied components. The system includes an additional heat exchanger to cool and liquefy the remaining non-liquefied components after the gas-liquid separator separates the liquefied component from the natural gas. The heat exchangers employ cold heat generated upon regasification of LNG. The system can reduce the heating value of natural gas composed of a variety of hydrocarbon components according to requirements of a place of demand by separating the component with the higher heating value from the natural gas to allow the separated component to be used as fuel, thereby reducing an overall size and operating costs of the system.