Tritium retention in carbon deposition is known to be a serious problem in a fusion reactor. In the present study, co-deposition behavior of carbon and hydrogen was investigated by using methane-hydrogen mixed plasma. It has been shown that formation of the carbon deposition was suppressed at temperatures higher than 150 °C. However, a small amount of carbon deposition was observed even at a high temperature of 300 °C. From mass balance calculation, it was estimated that a majority of carbon decomposed in the plasma was transported to the vacuum pumping system.