NOVELTY - The method involves introducing the mixed gas into the adsorption towers (10,20). The carbon dioxide gas by which the adsorption agent is adsorbed is desorbed under reduced pressure from the adsorption tower, and the adsorption agent is reproduced. The adsorption process is stopped based on the temperature change of the adsorption agent detected by the temperature sensor arranged among the temperature sensors at the discharge side of the purified gas. The density of the target gas in the purified gas obtained by the purifier (100) is turned into more than a desired density.    USE - Method for purifying mixed gas such as biogas used as town gas material and motor fuel for treating organic waste such as sewage sludge and foodstuff waste material, using purifier.    ADVANTAGE - The economical and efficient mixed gas can be obtained. The purified gas of consistent quality can be obtained stably. The misdetection of the stop of temperature fall resulting from the micro temperature change of the desorbed gas flowed through the inside of adsorption tower can be prevented. The operation cost of the purifier can be reduced.    DETAILED DESCRIPTION - The adsorption process is stopped based on the temperature rise and fall of the adsorption agent detected by the temperature sensor arranged at the discharge port side of the purified gas and desorbed gas respectively. The target gas is methane gas and the mixed gas is methane fermentation gas. An INDEPENDENT CLAIM is included for a purifier has an adsorption tower that is provided with the temperature measurement sensor that measures the temperature of the adsorption agent, and the air intake/exhaust ports (19a,19b). The mixed gas supply structure is made to supply the mixed gas to the adsorption tower through the air intake/exhaust port of adsorption tower. The impurity gas is desorbed by the impurity gas desorption mechanism from the air intake/exhaust ports. A control apparatus (80) is made to control the mixed gas supply structure and the impurity gas desorption mechanism. The temperature sensor is arranged in the nearest vicinity of the air intake/exhaust port out of which the purified gas is flowed out among the temperature sensors during the operation of mixed gas supply structure.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the purifier. (Drawing includes non-English language text)    Adsorption towers (10,20)    Air intake/exhaust ports (19a,19b)    Vacuum pump (71)    Control apparatus (80)    Purifier (100)