To provide a method for removing a heavy fraction in a decomposition gas for preventing the generation of an ultra heavy fraction caused by the polymerization of an olefin component contained in a gas liquid mixture of the heavy fraction condensed and liquefied in a mixing process with a gas, and a viscosity elevation of a circulating quench oil.

This method for removing the heavy fraction of an ethylene plant decomposition gas comprises mixing the decomposition gas from the ethylene plant with a circulating quench oil supplied from a fractionating process in a first mixing process to make a gas liquid mixture, separating the gas liquid mixture by feeding it to a gas liquid-separating process, feeding the liquid to a fuel oil-stripping process as desired, directly feeding the gas to the fractionating process or feeding to a second mixing process to form the gas liquid mixture cooled to the vicinity of a temperature at the bottom of a fractionating column in the fractionating process by mixing with the circulating quench oil supplied from the fractionating process, then feeding the mixture to the fractionating process to separate it to a light fraction and the circulating quench oil, removing the heavy fraction by feeding the light fraction to the quenching process, and feeding at least a part of the circulating quench oil to the first mixing process.