PURPOSE: To offer the apparatus being capable of detecting absolute humidity electrically and measuring it, by utilizing the property of the resonance frequency of a microwave band cavity resonator which varies for variations of a dielectric constant of inner gas.

CONSTITUTION: In order to detect frequency variations of microwave frequency band resonator, a semiconductor active element 12 is connected to a dielectric resonator 11 having a little frequency fluctuation to a dielectric constant of inner gas at first and a microwave signal is obtained. Frequency of this signal is nearly the same as the resonance frequency of the resonator 11 because the frequency of said microwave signal is stabilized by the resonator 11 and accordingly, the signal frequency difficult liable to the variation of the dielectric constant of surrounding gas is obtained. Next, the microwave signal obtained by the resonator 11 of a microwave signal oscillator 11 is introduced into a microwave band oscillator 2 consisting of a metallic electrically conductive wall such as a semicoaxial resonator or waveguide resonator and a cavity filled up with air. The variation of the resonance frequency is detected as the value of detection voltage of a detection circuit 3 by adjusting that the signal frequency of the oscillator 1 is found on a slope of the resonance curve of the resonator 2 at first because the resonance frequency of the resonator 2 is ralated functionally to the specific inductive capacity of the inner gas.