NOVELTY - A composition comprising methane, carbon dioxide, oxygen, nitrogen raw material gas, and other gases is supplied to a membrane separation unit, and carbon dioxide is separated. An intermediate gas obtained from the non-permeate side is temporarily stored. The intermediate gas containing methane, carbon dioxide, oxygen, nitrogen, and other gas, is sent to a pressure swing adsorption tower filled with a methane-concentration adsorbent, and the components are separated. Carbon dioxide is adsorbed, and methane (96-99%) is output from the tower top.    USE - Methane concentration.    ADVANTAGE - The method enables economical methane concentration with high purity, yield, and recovery rate, reducing the scale of pressure swing adsorption.    DETAILED DESCRIPTION - A composition comprising methane (60%), carbon dioxide (39%), and a small amount of oxygen, nitrogen raw material gas and other gases is supplied to a membrane separation unit which uses a membrane having differing permeability with respect to carbon dioxide and methane, and carbon dioxide is separated from the permeate side of the membrane. An intermediate gas obtained from the non-permeate side is temporarily stored in a buffer tank. The intermediate gas contains methane (82%), carbon dioxide (17%), and oxygen, nitrogen and other gas (remainder). The intermediate gas is divided into three and sequentially sent to a pressure swing adsorption tower filled with a methane-concentration adsorbent, and the components are separated based on the differing absorption capacity of the different components in the intermediate gas. Carbon dioxide is adsorbed, and methane is output from the tower top with a concentration of 96-99%. The regeneration gas in the middle of adsorption tower after 240 seconds of adsorption, is pressure-reduced and temporarily stored in buffer tank. The remaining gas is returned to the adsorption tower and pressure-increased to complete the regeneration process. An INDEPENDENT CLAIM is included for methane concentration device.