The object of the presently disclosed embodiment is to improve heat dissipation and an overall cooling efficiency to raise a peak oscillation output. To achieve the object, there is provided a coaxial magnetron having the following configuration: Around a cathode, vanes and an anode cylinder form an anode resonant cavity, and a cylindrical side body forms an outer cavity. An input side structure having an input part and an upper structure are joined to both ends of the cylindrical side body. One end of the anode cylinder is joined to the input side structure. A groove (or step) for adjusting the distance between the structures and at the both ends is provided, and the groove is joined to the other end of the anode cylinder.