PURPOSE: To prevent defective soldering, by locating a strap ring along a taper surface, made deeper to the side to be soldered, on the bottom of a strap groove of a vane to solder.

CONSTITUTION: Inside an anode cylinder 1, plural vanes 2 are placed radically, which are combined with a large and a small strap rings 3 and 4, to make a cavity resonator A to be a magnetron. In this case, as well as grooves 5 and 6 are formed on the vane 2, taper surfaces 10 and 11 made deeper to the side B to be soldered are furnished at the groove bottom where strap rings 8 and 9 are soldered. Then the rings 8 and 9 are pushed from outside by jigs 14 and 15 to squeeze to the side B along the taper surfaces 10 and 11 so as to locate and solder them. Therefore, gaps between the ring and the side can be avoided, defective soldering being prevented.