PURPOSE: To prevent high-frequency power from leaking by providing a cut-off waveguide at a wall surface portion of a heating chamber near a gas burner.

CONSTITUTION: Gas is fed to a pilot burner 9 and ignited by an igniter 10. The flame of the pilot burner 9 passes through a cut-off waveguide 8 and reaches the tip end of the pilot burner 9. A flame detector 11 detects the flame of the pilot burner 9, and a gas burner 5 is ignited. On the other hand, a door 14 is closed, a high-frequency oscillator 3 oscillates a high-frequency wave and a body to be heated 2 is heated by the high-frequency power. Since the pilot burner 9 and the cut-off waveguide 8 are welded and in a short-circuitted condition, the high-frequency wave cannot leak.