NOVELTY - Isotope selective adsorption agent contains sodium-A type zeolite comprising portion of sodium substituted with calcium or zinc by heat processing at 650-800degreesC.    USE - For selectively separating methane isotope such as 12CH4 from source gas concentrating 12CH3D (claimed).    ADVANTAGE - The isotope selective adsorption agent carries out separation concentration of methane isotope economically and efficiently, and the separation apparatus is compact.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) isotope-separation concentration method, which involves contacting source gas containing methane isotopes (12CH4 and 12CH3D) with adsorption group containing the adsorption agent, selectively adsorbing 12CH4, excluding 12CH3D, inducing the adsorption agent from the time of adsorption to adsorption completion to low pressure, removing 12CH4-enriched gas and regenerating the adsorption agent; and    (2) separation apparatus of methane isotope, which has 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 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 stored temporarily. 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 towers (31-33)    entrance-side flow path (A)    exit-side flow path (D)    counter-flow purge reproducing portion (C)    parallel current purge portion (D)