The invention relates to a device for the generation of microwaves, comprising a virtual cathode oscillator (1) in coaxial construction having an outer substantially cylindrical tube constituting a cathode (2) and connected to a transmission conductor (14) for feeding the cathode (2) with voltage pulses, as well as an inner substantially cylindrical tube, at least partially transparent for electrons, constituting an anode (3) and connected to a wave guide (13) for the discharge of microwave radiation generated by the formation of a virtual cathode (4) inside a region enclosed by the anode. The device comprises an electrically conductive structure in the form of a reflector (19) disposed adjacent to the anode (3). The cathode (2) comprises a substantially rotationally symmetric, electrically conductive body (15) having a cavity (16). By configuring the cavity (16) in the body (15) of the cathode with a first, lesser depth to that boundary surface (18) of the body which is directly in front of the peripheral part of the closure of the anode (3) against the cathode, and a second, greater depth to the boundary surface (17) of the body directly in front of the central part of the closure of the anode (3) against the cathode, a device for the generation of microwaves is produced, which has higher efficiency and high peak power.