PURPOSE: To decrease the fitting height of a magnetron by forming a permanent magnet into a predetermined shape and reducing the pressure loss when the forced air cooling air current passes.

CONSTITUTION: A permanent magnet 10a fitted to a magnetron used for a microwave oven range or the like is formed into a noncircular shape having the size L in the parallel direction larger than the size W in the vertical direction to the passing direction Wa of the forced cooling air current of a forced draft current passage 14, end the passage 14 is formed with the outside face of the size W and a frame-shaped yoke. According to this constitution, the pressure lose when the forced cooling air current passes is remarkably reduced as compared with the case that the magnet 10a is formed into a circular shape, the fitting height of the magnetron is decreased, and the microwave oven range can be made small-sized.