NOVELTY - The impurity in fuel gas (BG1) is condensed using cooling-heat of liquefied natural gas. The impurity treatment process is performed by using the impurity as solid state or liquid state material. The natural gas vaporized by heat exchange with fuel gas before the refinement process, is collected while separating the refined fuel gas (BG2). The impurity is condensed by fin surface after passing the liquefied natural gas into fin pipe of the heat exchanger (12).    USE - Purification method of bio-gas used for fuel cell, alkyl halide (RX) gas-evolution installation and gas-engine generator.    ADVANTAGE - The impurities such as carbon dioxide in fuel gas, moisture content, oil component and methane are removed effectively. The purity level of hydrogen and carbon monoxide and methane which are the structural components of fuel gas can be increased.    DETAILED DESCRIPTION - The resurgent gas (RG) is contacted to impurity in solid state or liquid state, for vaporization. The regenerated gas from the refined fuel gas is removed from fin surface. The temperature of refined fuel gas is measured. The flow volume of fuel gas before refinement, and flow volume of liquefied natural gas is adjusted so that the measured temperature is fixed. The product gas is manufactured by mixing the natural gas after vaporizing the impurity, and fuel gas. An INDEPENDENT CLAIM is included for fuel gas purification system which includes heat exchanger having fuel and natural gas flow channels connected at heating surface. The fuel gas and liquefied natural gas are passed through corresponding introducing sections (110,210), and are extracted through corresponding leading-out sections (150,250).    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view illustrating the structure of fuel gas purification system. (Drawing includes non-English language text)    Fuel gas purification system (1)    Gas mixing adjustment system (2)    Impurity process section (11)    Heat exchanger (12)    Fuel gas introducing section (110)    Fuel gas leading-out section (150)    Liquefied natural gas introducing section (210)    Liquefied natural gas leading-out section (250)    Fuel gas (BG1)    Refined fuel gas (BG2)    Resurgent gas (RG)