Provided are an apparatus and method for measuring a dielectric constant. The apparatus includes a cavity resonator including a cavity therein, an insertion hole penetrating through the cavity vertically and in which a sample is inserted, and grooves symmetrically formed with respect to the cavity in the insertion hole, a network analyzer configured to generate an electromagnetic signal supplied to the cavity resonator, receive an electromagnetic signal passed through the cavity resonator, and calculate a scattering parameter, a transmission means configured to supply the generated electromagnetic signal to the cavity resonator, a reception means configured to supply the electromagnetic signal passed through the cavity resonator to the network analyzer, and a calculation processor configured to receive the scattering parameter from the network analyzer and calculate a dielectric constant of the sample.