PURPOSE: To obtain a magnetron having a stable operation mode while making it possible to lower its cost, to reduce its size and to thin its shape by providing windows for emitting microwaves on the anode peripheral side with a period synchronizing with a desired microwave resonance mode.

CONSTITUTION: An opening part is provided about in the middle part in height on the peripheral surface of the anode cylinder in 180° symmetry and by the length of an arc containing an angle of 90°. These openings are the coupling windows 13. As for a vacuum gastight construction, the whole anode part is housed in a cylindrical container made of alumina porcelain. For the operation of the magnetron, a space between the cathode and a comb-formed construction 12 requires being vacuum so that electrons may be able to move in the space. A gastight separation wall made of a dielectric material may be provided inside the anode container. A mode of a degree matching to the positions of the windows inside a cavity is stably excited and its microwave power can be taken out through the coupling windows as an output. Thus a magnetron having a stable operation mode, a small size, a thin form and of low production costs is obtained.