To simplify a microwave generating device by providing a first grid having multiple holes for controlling and focusing the electron flow from a cathode on a heating element and a first secondary electron emission decreasing means, a choke structure, an input cavity, a bias voltage resistor, a second grid, an anode having a second secondary electron emission decreasing means and an output cavity.

Part of the electrons from a cathode 120 on a heater 110 are converted into electron beams by the holes 135 of a first grid 130 and reach an anode 150 through the holes 145 of a second grid 140. The electron beams accelerated by the electric field between the input cavity 170 of a resonance circuit formed with the first grid 130 induced with a bias voltage, a choke structure 160 serving as a blocking capacitor and the cathode 120 and an output cavity 180 formed with the second grid 140 and the anode 150 are outputted as microwaves. No magnet is required, and a metal film is provided on the first grid 130 and multiple projections are provided on the anode 150 to decrease the emission of secondary electrons impairing the electron flow.