PURPOSE: To prevent dew condensation at a humidity sensing agent and electrodes by raising the temperature of a substrate when the humidity is larger than a specific value.

CONSTITUTION: The humidity sensing agent 3 manufactured by polymerizing methacryl amide propyl trimethyl ammonium chloride as a cation conducting agent monomer and vinyl acetate is applied between gold-made comb-shaped electrodes 2 and 2 printed on the surface of an alumina substrate 1, and a heating body 4 for prevention against dew condensation which consists of an electric heater is embedded in the reverse surface of the alumina substrate 1. When the relative humidity rises above 90%, the heating body 1 is powered on to heat the substrate 1, and this heat conduction makes the humidity sensing agent 3 and electrodes 2 and 2 warm enough to prevent dew condensation, reducing the influence of the dew condensation.