A magnetron has an anode cylinder, a plurality of vanes arranged radially within the anode cylinder, a magnetic piece disposed at an open end section of the anode cylinder, an anode vacuum container including a metal container disposed to cover an upper surface of the magnetic piece, a cathode disposed along a central axis of the vacuum container, and an antenna externally discharging microwaves. The magnetic piece and the metal container are placed, in that order, on a shelf formed inwardly on a thin end section projecting from the open end section of the anode cylinder. When tightly welding the thin end section, a predetermined number of projections, projecting inwardly from the thin end section of the anode cylinder, loosely secure an outer perimeter bend of the metal container. The metal container is then accurately tightly welded to the anode cylinder without the metal container shifting off-center.