Abstract: A measuring apparatus and method of a permittivity of a dielectric material employs a cavity and a variable reactance connected to the outside the cavity to measure the permittivity of the dielectric material, in which a power-feeding post is installed within the cavity, the variable reactance for forcibly resonating the cavity is attached to the upper portion of the cavity at the end of the power-feeding post, a network analyzer is connected to the power-feeding post, and the dielectric material specimen to be measured fills up the cavity, so that the length of the variable reactance is adjusted to forcibly resonate the cavity and, at this time, the length of the variable reactance is measured to determine the permittivity of the dielectric material. Thus, unknown permittivity is simply measured while enabling the broadband measurement. A small cavity at a cut-off state is employed to be suitable for measuring the permittivity in a low frequency band.