To form light gravity the liquefied natural gas in LNG tank to reduce its heating value.

This device is provided with an LNG tank 1 for storing a liquefied natural gas, a heater 9 for heating one part of the liquefied natural gas sent from LNG tank 1 to a carburetor 8, a decompressor 10 for decompressing the heated liquefied natural gas, a flash drum 11 for separating a light gravity gas with a low heating value from the liquefied natural gas to the vaporized light gas and the heavy gravity gas with a high heating value of a liquid by heating and decompressing, a condenser 7 for returning the light gravity gas to LNG tank 1 after liquefying by the cool heat of the other liquefied natural gas sent from LNG tank 1 to the carburetor 8, and lines 5, 6 for sending out the heavy gravity gas to the carburetor 8 after mixing to the other liquefied natural gas liquefied the light gravity gas so as to be in an allowable heating value.