In this study, natural gas was used as raw material and enriched gas of carrier gas, and the development of the system which shortened the carburization time was examined. The mechanical property of the sample which is treated by using this method was evaluated. The control of carbon potential by the oxygen concentration is possible, if enriched gas flow rate in which soot does not arise is chosen, even if the natural gas is used for enriched gas. And, the responsibility is better than carbon potential control by the carbon dioxide concentration. Using the difference between carbon potential calculated from carbon dioxide concentration and and it calculated from oxygen concentration, the system which foresaw the soot generation was developed. By controlling the carbon potential using this system and oxygen sensor, the carburization time was shortened, and retained methane quantity and using enriched gas quantity are drastically decreased.