A dielectric constant detecting apparatus comprises an LC circuit for shifting the phase of a high frequency voltage signal according to the dielectric constant of a liquid which passes between the coil and the electrode of the LC circuit. A resistor is connected between the LC circuit and at least one part of a phase comparator. The phase comparator detects the phase shift between high frequency signals provided at both ends of the resistor. The resistor and at least one part of the phase comparator are integrally included in an insulator. The output of the phase comparator is compared with a predetermined phase shift value. A voltage control device adjusts the high frequency voltage signal from the voltage applying device so that the phase shift detected by the phase comparator is further adjusted toward the predetermined phase shift value. At least one of the voltage applying device and the voltage control device produces a signal based on the phase shift which is representative of the dielectric constant of the liquid.