There is provided a thermal therapy device that is capable of following changes in the resonance frequency even if the resonance frequency of the series resonance circuit on the secondary side changes significantly and generating a magnetic field of a required intensity for heating the affected portion. The thermal therapy device comprises an inverter circuit (2) connected to the primary side of a transformer (1), a series resonance circuit (5) connected to the secondary side of the transformer and including a magnetic field generation coil (3) and a resonance controller (4), and a controller (20) for determining the resonance frequency based on the secondary current. The therapy operation mode may be a measurement mode and an operation mode which are alternately repeated. During the measurement mode the resonance frequency of the series resonance circuit (5) is measured, while during the operation mode a drive signal of said measured resonance frequency is supplied from the controller (20) to the inverter circuit (2). Accordingly, the frequency of the drive signal to be supplied to the inverter circuit can follow the resonance frequency even if the impedance of the series resonance circuit (5) or the like varies during therapy.