PURPOSE: To have optimum impedance matching of a magnetron of a short output end type with a waveguide pipe in conventional dimensions by specifying the distance of the center axis of the output end from a short plate of waveguide pipe, and arranging an impedance matching stab within waveguide pipe leading from the output end to the load side.

CONSTITUTION: The distance of the center axis of an output end 2 from a short plate 3 of a waveguide pipe 4 is set to 0.1-0.2λ (λ is fundamental wavelength of oscillated microwave). An impedance matching stab 15 is arranged in the waveguide pipe 4 leading from the output end 3 to the load side. This provides optimum impedance matching owing to reflecting action by the short plate and stab even though a magnetron of short output end type is used in combination with a waveguide pipe is conventional dimensions, and common use of waveguide pipe or interchangeability of magnetron is ensured.