PURPOSE: To improve the thermal efficiency of a microwave oven by providing a magnetron which has a vacuum vessel made of a material which is transmissive to hot ray energy for a microwave oven.

CONSTITUTION: A table 53 receiving a heating object 52 is provided at the bottom of a microwave oven 51. A funnel-shaped wave guide 54 is provided at the central part of the upper part of the microwave oven. A magnetron 10 is provided near the end face of the wave guide 54. The vacuum vessel 17 of the magnetron is made of a material which is transmissive to microwave, infrared ray and visible light to make the thermal radiation caused by the anode loss irradiate the heating object 52 together with the microwave. Accordingly, the thermal efficiency of the microwave is improved as the thermal radiation is utilized in addition to the radiation by the microwave.