PURPOSE: To obtain a plurality of heating modes for foodstuffs to be cooked and prevent a spark in a waveguide from occurring and an impedance from being varied.

CONSTITUTION: An opening 18 is formed in a back plate which is a wall of a heating chamber 4, and a disc 23 which is driven for rotation by a motor for the disc is disposed in the opening 18. An opening 23a is formed in the disc 23 and is selectively located in an upper position or in a lower position by driving the disc 23 for rotation. A microwave from a magnetron 29 passes through a waveguide 28 and a cavity box 19 and is radiated into the heating chamber 4.