Abstract: A dielectric constant sensing device of the present invention comprises a transmission line consisting of a first electrode formed of a conductor being wound like a longitudinal cylinder, a second electrode provided to be separated from the cylindrical surface of the first electrode at a predetermined distance, and a chamber for introducing a measured fluid between the first electrode and the second electrode, whereby a dielectric constant of the measured fluid can be sensed based on a rate of a pulse voltage wave propagated over the transmission line. Accordingly, the fluid dielectric constant sensing device which is able to attain high precision by a simple structure can be provided.