Tin oxide is a non-stoichiometric N-type semiconductor, and its electrical conductivity is largely influenced by the chemisorption of oxygen and hydrogen. We studied the chemisorption phenomena and its states by the variation of the electrical conductance, so that we first had to decide the conduction mechanism. The sample was composed of numerous crystallites and their surfaces were covered with the oxidized film, on which the negative oxygen ions were chemisorbed. Hence they formed the potential barriers for conduction. Oxygen chemisorption caused to not only reduce the carriers but also increase the hight of potential barrier. On the contrary, hydrogen chemisorption caused to increase the carriers, reducing the barrier height. Therefore, both the electrical conductance and the chemisorption velocity were largely influenced by the chemisorption, even if the coverage was very small. Moreover, the heat of adsorption influenced the electrical conductance, especially at high temperatures.