Oxygen adsorption on the surface of particles plays a key role in gas sensing for SnO2-based resistive-type gas sensors in a humid atmosphere. In this study, we added an extremely small amount of ZrO2 on the SnO2 surface to activate oxygen adsorption and enhance the sensor response of a SnO2-based gas sensor in a humid atmosphere. We evaluated the oxygen adsorption properties and sensor response to 200ppm H-2 in a humid atmosphere (96%) based on variations in the measured electrical resistance. The adsorption of O- and O2- ions was activated by a small loading (0.033mol%) of ZrO2, and the resulting ZrO2-SnO2 composite nanoparticles were in the deep electron-depleted state. This led to a high sensor response of ZrO2-SnO2 to H-2 in a humid atmosphere. The results demonstrate that surface modification using an extremely small amount of ZrO2 was effective in improving the response of a SnO2-based gas sensor in a humid atmosphere.