PURPOSE: To surely perform welding of a york by plurally boring the bottom surface of a filter case, i.e. the contact surface side to a yoke.

CONSTITUTION: In a filter case 5, a plural number of bores 7 are provided on the bottom surface 5 of a filter case 5 abutting against the input side 1' of a yoke 1. Thereby, when a welding is performed by means of welding electrodes 8, an electric conduction 8 is made in a state that contact points of welding electrodes 8 laps over the circumferential parts of bores. Thus, a strong welding effect is obtained as an iron material 6 of welded parts 6 is fused to the inside, so that welding of the yoke 1 is surely performed and as a result, a highly reliable magnetron is obtained.