Provided is a low-cost magnetron that is excellent in productivity without any adverse effect on characteristics.Two large and small strap rings 11 (11A and 11B) are only disposed at lower ends, or input sides, of a plurality of vanes 10 (10A and 10B) in the direction of a tube axis m. Diameter Rip of a protruding flat surface 41 of an input side pole piece 18 is larger than diameter Rop of a protruding flat surface 40 of an output side pole piece 17. Therefore, it is possible to provide a practical magnetron without a significant decrease in productivity or characteristics from a conventional one, while cutting costs by reducing the number of parts with the use of two strap rings on one side.