This paper describes a simple thermoelectric psychrometer that measures equilibrium humidity of a sample in a small vessel. The psychrometer consists of one thermocouple and detects the wet-and dry-bulb temperatures using the same measuring junction. The detector consists of a K-thermocouple, 0.1mm in diam., and the surface of the junction is covered with a thin fiber material. When the fiber is moistened by water, the junction acts as a wet bulb. As the moisture in the fiber is lost, the temperature of the junction rises rapidly to that of the dry bulb. The psychrometer constant is derived from equi-dew-point lines drawn up experimentally and is found to be 0.00074°C-1 Data of the equilibrium humidities observed for saturated sodium chloride solution and some foods are shown.