PURPOSE: To obtain the uniform distribution for the magnetic flux density in the active space by setting the ratio to 1.0W1.4 between the diameters of the magnet and the pole piece for the single magnet type magnetron.

CONSTITUTION: For the single magnet type magnetron containing a magnet at the side where no cathode lead exists, magnet 7 is fitted and fixed to sealing wall 5 from outside. And pole piece 8 of one side is attached to magnet 7 via holding component 5. Pole piece 9 of the other side is provided at the lower part of cathode 1 from the lower part of cylindrical substance 4. Then the magnetic field is applied concentrically to active space 10 between cathode 1 and anode fin 3 via pole piece 8 and 9. Here, the ratio is set to 1.0W1.4 between diameter D1 of magnet 7 and diamter D2 of pole piece 8 in order to secure the uniform magnetic flux density in active space 10. Thus, the dispersion of the magnetic flux density can be reduced less than 10%.