PURPOSE: To reduce the size of an area which cooling air strikes and improve cooling capacity by providing a concave cross-sectional shape for a yoke located on the cooling air input side of a longitudinal blow type magnetron which makes the cooling air strike in parallel to the axis.

CONSTITUTION: Ring-shaped permanent magnets 2 and 3 are overlapped centering the bulb main body 1 of a magnetron and a magnetic circuit is formed with a frame-shaped yoke 4 in a configuration to hold them between, while a cooling fin 5 with a radial blade section is provided outside the bulb main body 1. Besides, a filter box 6 is mounted on the input side and the bulb main body 1 is cooled by allowing cooling air to enter a duct 7. 43a on the lower plate 43 of the yoke 4 hit by cooling air is molded in a concave shape so that the width W1 of the section 43a can be reduced. As a result, a great amount of cooling air can be supplied and the cooling capacity of the magnetron and its reliability can also be improved.