To provide a magnetron which suppresses microwave signals leaking from a negative electrode and operates with stability.

In a magnetron provided with choke coils 13a, 13b which consist of core parts 14a and air-core parts 15a, 15b and are connected with negative electrode terminals 12a, 12b, capacitors 17a, 17b connected to the choke coils 13a, 13b and a filter box 11 where the choke coils 13a, 13b and the capacitors 17a, 17b are stored, insulating members 18a, 18b fixed to air-core parts 15a, 15b of the choke coils 13a, 13b and mobile mechanisms 20a, 20b, which moves the insulating members 18a, 18b in the direction where the choke coils 13a, 13b are extended, are provided.