A ferromagnetic resonance measuring cavity resonator comprises a cavity resonator having a cavity formed by a side wall. The side wall is provided with an input/output hole and a through-hole. A ferromagnetic plate provided with a through-hole is mounted on the outside surface of the side wall such that the two through-holes are aligned. A wafer specimen is pressed against the ferromagnetic plate to face the through-hole. When microwaves are introduced into the cavity resonator, ferromagnetic resonance takes place based on the static magnetic field applied to the wafer specimen and the microwaves. As a result, a ferromagnetic resonance signal is detected via the input/output hole.