The present study introduces a humidity control apparatus which can hygienically and automaticlymaintain the appropriate humidity level of indoor air without either a water supply or drainage system.In humidification, the desiccant takes up water vapor from the outdoor air in the adsorption process,and releases it indoors by desorption. Therefore, no water supply is required to humidify. An apparatus havingno water supply is more hygienic, because it does not propagate bacteria and does notscatter the calcium and magnesium salts found in the water. In dehumidification, water vapor is removedfrom the indoor air by the desiccant and is released outdoors by resorption, eliminating theneed for drainage system. The absence of a drainage system also eliminates such problems as waterleakage and installation. An adsorbent was expected to adsorb extremely little amount of formaldehyde,toluene and xylene in the indoor air. An experiment, therefore, was made to discharge volatileorganic compounds (VOCS) outdoors in the resorption process after adsorbing them in the adsorptionprocess. It is possible, therefore, that indoor air pollution due to such VOCS can be prevented on adaily maintenance-free basis.