This research is performed to determine how gaseous impurities affect the evacuation of tritium from a fusion reactor chamber by using a cryosorption pump. The amounts of H2O and CH4 remaining on activated carbon during repeated cycles between adsorption for evacuation and desorption for regeneration are related to the partial pressures of the H2O and CH4 and to temperature and are correlated in terms of Henry's law. It is experimentally investigated how the impurities remaining on the activated carbon after rough evacuation by elevating the temperature affect the adsorption of H2 and He at cryogenic temperature. The amount of CH4 remaining on activated carbon is smaller than that of H2O, and it was found that the former's effect is comparatively smaller than the latter's. Discussion is made based on the surface coverage and pore distribution that are determined by the BET measurement. An important conclusion is that the bulk of the H2O and CH4 can be released from activated carbon by heating to 373 K.