PURPOSE: To prevent damage of an inner frame when a material to be heated is output and input by using a packing to be used to seal a door from a heating chamber.

CONSTITUTION: A sealing packing 15 protrudes from a surface of an inner frame 7, and has a hollow sealing part 17 in contact with a front plate of a heating chamber. A synthetic resin frame 11 is formed with a protrusion 18 disposed on a rear surface of the part 17 of the packing 15 to accelerate the displacement of the sealing part at the time of bringing the packing 15 into contact with the front plate of the chamber, and a step 19 connected to the protrusion 18 to contain a compressively deformable part of the part 17. Thus, the protruding size of the part 17 can be increased without loss of sealability for leakage of a radio wave to perform a role of protection of the frame 7 when a material to be heated is input or output by the part 17.