NOVELTY - A raw material containing hydrogen, methane, and 2C or more hydrocarbon is supplied to a multistage vapor-liquid separation tank (31) and separated into gaseous-phase component and liquid-phase component. A portion or whole of nitrogen oxide (NOx) containing light gas and liquid-phase component are supplied to a distilling column (62), distilled and separated into liquid-phase component and gaseous-phase component. The gaseous-phase component is supplied to a demethanizer (61), and 2C or more hydrocarbon fraction is separated from NOx containing light gas.ethylene and propylene    USE - For separating 2C or more hydrocarbon fraction containing olefin fractions such as ethylene and propylene from nitrogen oxide-containing light gas produced in petroleum refining process.    ADVANTAGE - The method separates 2C or more hydrocarbon fraction efficiently and economically, without increasing load of demethanizer, and suppressing production and deposition of nitrogen oxide and fluctuation of flow rate of nitrogen oxide-containing light gas and composition, temperature and pressure of demethanizer.    DETAILED DESCRIPTION - A raw material containing hydrogen, methane, and 2C ore more hydrocarbon is supplied to a multistage vapor-liquid separation tank (31-34) and separated into gaseous-phase component and liquid-phase component. The gaseous-phase component is separated into hydrogen fraction and low-pressure methane fraction in hydrogen-low-pressure methane separation tank (51). The liquid-phase component is distilled in a demethanizer (61), and 2C or more hydrocarbon fraction is separated as liquid-phase component. The gaseous-phase component from cooling tower top of demethanizer is separated into high-pressure methane fraction and medium-pressure methane fraction. The remaining product is recirculated to demethanizer by a recirculation tank (71). The hydrogen fraction, low pressure methane fraction, medium pressure methane fraction and high pressure methane fraction are cooled with a cryogenically cooling heat exchanger (41) by exchanging heat of gaseous-phase component of multistage vapor-liquid separation tank. A portion or whole of nitrogen oxide (NOx) containing light gas and liquid-phase component separated by vapor-liquid separation tank is distilled in a distilling column (62) and liquid-phase component and gas-phase component are separated. The gaseous-phase component is supplied to the demethanizer, and 2C or more hydrocarbon fraction is separated.    DESCRIPTION OF DRAWING(S) - The figure shows the structure of the cryogenically cooling system provided in circumference of the demethanizer.    Cryogenically-cooling system (1)    Multistage vapor-liquid separation tank (31-34)    Cryogenically-cooling heat exchanger (41)    Hydrogen-low pressure methane separation tank (51)    Demethanizer (61)    Distilling column (62)    Recirculation tank (71)