PURPOSE: To enable safe oscillation at a required mode so as to improve a cure effect by providing a good conductor outer shell forming a hollow resonator and a high resistance area with a specified pattern formed at the inner face of the shell.

CONSTITUTION: A good conductor outer shell 1 is formed in a cubic form and is provided with at its side, patient insertion holes 2, 2. On the inner face of the shell, a high resistance area 3 with respect to a high frequency current is formed with a specified pattern. This pattern serves to generate a TE321 mode. At the time of the TF123 mode, the distribution of electric fields is settled in the figure A wall surface current flows along the inner face of the shell with a pattern in response to the distribution although, as the pattern of the area 3 is so formed as to be in response to the pattern of the current, an inner wall surface current with another pattern is restrained in its generation. As a result, only the TE321 mode inner wall current flows, resulting in generation of only TE321 mode.