PURPOSE: To detect water content in material free from effect of steam, atmospheric temperature and air current included in the atmospheric nature by heating a humidity detection element and material to be detected with a heater.

CONSTITUTION: A heater 4 is maintained constant with material to be detected by a position controlling means 8 and energized to heat the surface of the material. Steam generated from the material being detected gets into contact with a humidity detection element 1 passing through an explosion proof metallic screen 5. The quantity of the steam generated is determined by the water content of the material being detected by keeping the heating temperature of the heater 4 constant while the ambient air current is cut off with a cover 6. This enables a highly accurate detection of trace of water free from the effect of the ambient temperature and air current and steam included in the atmospheric nature.