Hydrogen permeation measurements were carried out with Ag 23wt% Pd alloy membrane of 20μm in thickness so as to clarify the effect of co-existing gases on hydrogen permeability through the steam reforming reaction process. Co-introduction of impurity gases such as H2O, CO2, CO, CH4 with hydrogen did not affect hydrogen permeability of palladium alloy membrane at high temperatures (773K, 873K). The rate determining step was the bulk diffusion of H in the membrane. However, effects of co-existing gases were visible at low temperatures in H2-H2O, H 2-CO, H2-CO2, H2-CH4 atmospheres. Rate equation of surface reaction was obtained from the result of hydrogen permeation measurements. The results of performance simulation by the use of surface reaction rate equation suggested that the effect of impurity gases on hydrogen permeability was negligibly small at high temperatures.