A unique method to measure the sticking probability of H2O in chemical adsorption has been developed. It uses molecular beam detection in which a mass filter with differential pumping can only detect H2O released from the sample surface. First, the amount of the dose is measured from reflected H2O during its irradiation to a sample surface. Next, the amount of adsorption is measured from released H2O by heating the sample. The sticking probability of H2O is obtained through the ratio of these amounts. The measurement limit at one Langmuir dose is 1 X 10(-4). Error may be expected to be within 100%. We apply this method to an SUS304 surface to measure the dependence on the amount of adsorption. This suggests that there are two kinds of adsorption sites.