PURPOSE: To fix easily and firmly while to improve cooling effect of spiral fin by performing relative positioning between yoke and magnetron through a positioning board.

CONSTITUTION: Contour of positioning board 8 is such that diameter D will fit to the inner diameter D of circular rib 12 of inner face and fixed. Diameter a of central hole 9 of said board 8 is approximately same with diameter a at the high frequency output section 7 of magnetron body and when fitting said section 7 in said hole 9, relative position against a yoke of high frequency output section is fixed firmly through the positioning board. A rising 10 is provided at the circumference of said board 8 to choke a gap against a spiral fin 1 at the corner of shoulder of yoke 5a. Since cooling water must pass in the way of spiral fin 1, heat source or an anode tube 2 is cooled efficiently.