NOVELTY - A selective adsorption agent, which selectively adsorbs methane isotope (12CH4) among isotopes (12CH4,13CH4,12CH3D) of methane gas is contacted with methane gas. Isotope (12CH4) is adsorbed to adsorption agent, and gas excluding isotope (12CH4) is recovered. Adsorption agent is induced from time of adsorption to low pressure, such that isotope (12CH4)-enriched gas is desorbed, and adsorption agent is regenerated.    USE - For separating methane isotope from high-pressure source gas, used for producing carbon isotope (13C) used in medical field and nuclear power industry.    ADVANTAGE - The separation method economically carries out separation concentration of methane isotope, and the separation apparatus is compact.    DETAILED DESCRIPTION - An adsorption group containing selective adsorption agent, which selectively adsorbs a methane isotope (12CH4) among methane isotopes (12CH4,13CH4,12CH3D) contained in high-pressure raw material methane gas, is contacted with the methane gas. The isotope (12CH4) is adsorbed to the selective adsorption agent, and gas excluding the isotope (12CH4) is flowed and recovered. The selective adsorption agent is induced from the time of adsorption to low pressure, such that the isotope (12CH4)-enriched gas is desorbed, and the selective adsorption agent is regenerated. An INDEPENDENT CLAIM is included for separation apparatus of methane isotope, which has an adsorption tower (31-33) filled with the selective adsorption agent, an entrance-side flow path (A) connected to the entrance side of the adsorption tower, an exit-side flow path (B) connected to the exit side of adsorption tower, a reproducing portion (C), and a parallel current purge portion (D). The entrance-side flow path introduces the high-pressure source gas to the adsorption tower, and the exit-side flow path recovers enriched gas excluding the isotope (12CH4). The reproducing portion flows an inert gas from the tower exit side, when the adsorption tower is adjusted to low pressure, and the isotope (12CH4)-enriched gas is desorbed. The isotope (12CH4)-enriched gas after desorption is temporary stored. The purge portion supplies the isotope (12CH4)-enriched gas to the entrance side of adsorption tower.    DESCRIPTION OF DRAWING(S) - The figure shows the isotope separation concentration apparatus (Drawing includes non-English language text).    adsorption tower (31)    entrance-side flow path (A)    exit-side flow path (D)    counter-flow purge reproducing portion (C)    parallel current purge portion (D)