A microwave generator, which includes a housing having two mutually opposite electrodes which are separated via an electrode intermediate space which is filled with a dielectric, and with the electrodes having a spark gap between them, which breaks down in order to emit microwaves when a high voltage is applied. The electrode intermediate space (13, 13a, 13b) can be at least partially filled with a second dielectric (15, 15a, 15b), which has a different dielectric constant compared with that of the first dielectric (14, 14a, 14b) which is located between the electrodes (6, 7) in order to vary the microwave frequency, whereby the second dielectric (15, 15a, 15b) is held in a reservoir (17, 17a, 17b) which communicates with the electrode intermediate space (13, 13a, 13b).