Abstract: A heating apparatus suitable for use in medical heat treatment of an affected portion of a human body employs a cavity resonator supplied with high-frequency energy from a high-frequency power supply unit. The cavity resonator comprises a cavity formed of a conductive material and at least one inner protrusion in said cavity generating a standing wave electric field for heating a body or body part introduced into the cavity. The inner protrusion may be a separate conductor or may be formed by deforming a portion of the conductor forming the cavity. A highly concentrated standing wave electric field and a surrounding electromagnetic field are produced in the cavity resonator when it is energized from the high-frequency power supply unit. The concentrated standing wave electric field intensively heats a desired local portion of the body and deep parts of the body may be heated satisfactorily.