This paper describes an improved apparatus to measure dielectric constants and dissipation factors using a resonance technique. The measurements can be made rapidly and accurately without appreciable systematic error in the wide frequency range from 1 kHz to 100 MHz. This apparatus uses the half power‐gap variation method or the half power‐varying gap‐immersion method for solid‐plate specimens and the solid‐plate displacement method for liquid specimens. The electrode system is 30 mm in diameter and has a shield ring to avoid the edge capacitance effect, guards against surface current leakage, and variable air capacitors to set the electrode gap at any arbitrary value. To detect a very small power loss, the resonance voltages in circuits withQ&bartil;100 are stabilized to −86 dB at a constant ambient temperature. We use phase detectors so that the electrode gap and the voltage at resonance can easily be read from the digital panel meter and the digital voltmeter. No corrections are necessary even at a high frequency of 100 MHz. The accuracy of measurements made using the half power‐varying gap‐immersion method is within 0.2% for the dielectric constant and 3% for the dissipation factor without marked degradation over the entire frequency range.