The hyperthermia as one of low-invasive therapies was taken notice of in aging society. There is a magnetic generating coil system, an applicator, as one research issue of the engineering developments in the medical treatment system. The proposed applicator system with the wireless transmission can be set a patient to operating bed easily and the gap between two flat coils can be adjusted to the breast thickness of a patient. But on the other hand, the distance between coils changes a mutual inductance and influences the characteristics of circuit and magnetic fields. This paper estimates the characteristics on the variation range of electric parameters by change of the size of exciting coils, and proposes the control methodology of the parameter fluctuation by a control of exciting frequency. The feasibility and performances of the apparatus were discussed by the simulation and the experiment.