PURPOSE: To make a magnetron surely and stably lockable on a frame inside a heater, by rotating and screw-propelling a nut being pressed in a rodlike projection bulging from a yoke plate and then screwing the nut in the projection tight enough.

CONSTITUTION: A yoke plate 7 constituting a magnetic circuit for a magnetron has a hollow, rodlike projection 8 being bulgingly formed in time of its own press forming, and at the vicinity of this rodlike projection 8, there are provided with escape holes 9 in order to secure the material required for the bulging form of the projection 8. The rodlike projection 8 is made up in form as bending a band into a U-shaped from and, after being inserted into a through hole of a frame inside a heater, machined for thread cutting by a nut 10 being pressed in its tip part. This nut 10, to say the least, is made up of a hard material in its screw thread part and given turning force directed clockwise while keeping the said pressed-in state of screw-propelling so that the nut 10 is closely screwed in with the rodlike projection 8 tight enough. Since the rodlike projection 8 is of hollow type, it diminishes in diametral size with the nut 10 screwed, but in combination with its own resilience, a close screw-tightening effect can be secured.