In such a case that a radial dimension of an outer circumference of a small-diameter strap ring of a magnetron is equal to "Rs1", a radial dimension of an inner circumference of a large-diameter strap ring is equal to "Rs2", a radius of a circumference which is inscribed to tip portions of anode vanes is equal to "Ra", and a radius of a central flat portion of a magnetic piece, which is located in the vicinity of each of the anode vanes, is equal to "Rp", the respective values of Ra, Rs1, Rs2, Rp are set in such a manner that the below-mentioned formulae (1) and (2) can be established: 1.85Ra ≦ (Rs1+Rs2)/2 ≦ 1.96Ra Rs1