Na clusters are generated in zeolite FAU with Si-to-Al ratios of 2.8 (Y-type) and 1.25 (X-type). When the density of adsorbed Na atoms increases, the ESR spectrum with hyperfine structures becomes narrow into a single line. In the IR absorption spectrum, no Drude-like absorption is observed at any adsorption densities of Na atoms. The ESR narrowing is explained by the mean field theory of the exchange narrowing given by Anderson and Weiss, by using the experimentally observed antiferromagnetic exchange coupling energy between magnetic moments of clusters.