PURPOSE: To obtain a compact water content detector in a simple configuration which can detect water content constantly with nearly a fixed sensitivity regardless of the fluctuation of the temperature of external air.

CONSTITUTION: A PTC thermistor 5 is mounted to the rear surface of an insulation substrate which is provided on the surface of a pair of electrodes 4a and 4b and a power-supply part 20 for applying a constant voltage is provided at the PTC thermistor. Since the PTC thermistor operates as a constant- temperature heating element, the temperature of a water content detection part is nearly maintained to be constant, thus eliminating change in water conductivity due to change in the temperature of an external air and detecting the water content constantly with a certain sensitivity.