For an anode assembly 51 of a magnetron, a plurality of plate shaped vanes 54 radially arranged at an inner circumference of the roughly round shaped anode assembly 53 has a end portion arranged at a central axis of the anode assembly 53 with a step shape Df having a reduced thickness in a range of predetermined length L from an end portion, so that increase of the facing area of the respective adjacent plate shaped vanes 54 is suppressed while the separation distance of the end portions of the vanes is secured.