The performance of H2O-PSA with Zeolite 3 A as adsorbent was examined for the dehumidifing of flue gas at the first stage of a CO2 separation system with CO2-PSA and cryogenic separation. Zeolite 3 A displaced with K+ at the site of the hydrogen ion has shown performance higher than active alumina with respect to the decrease in loss of CO2 in flue gas through H2O-PSA as well as the dehumidification of flue gas. In addition, it has been shown that there is a possibility of operating the H2O-PSA with low power by applying the purge with off-gas from CO2-PSA.