Abstract: In a microwave tube of the type wherein an electron beam emitted by an electron gun is focused and caused to interact with an input high frequency wave at a high frequency circuit unit by magnetic flux generated by a permanent magnet type magnetic circuit, and the electron beam after the interaction is collected by a hollow cylindrical collector, a leakage flux generating means is provided near the entrance of the collector for generating leakage flux acting upon the electron beam. The leakage flux is produced by providing a notch for a pole piece adjacent to the collector and connected to the high frequency circuit unit or by reducing the cross-sectional area of the pole piece for magnetically saturating the pole piece itself or by keeping the pole piece in contact only with a portion of one end of the permanent magnet.