A relativistic magnetically isolated vircator with a magnetic mirror on a prelimit electron beam is proposed. Its computer simulation has been carried out. The phase dynamics of an electron beam in a vircator has been studied. It is shown that a number of virtual cathodes appear in the beam after the beam is reflected from the magnetic mirror. The output microwave characteristics are calculated: the average power and the spectral composition of generation, containing a set of narrow spectral lines and their harmonics. The effect of the mirror ratio on the average output power and on the frequencies of the spectral lines is studied. It is found that the power increases with the growth of the mirror ratio. The frequencies of some spectral lines increase with the mirror ratio, while the frequencies of other lines do not depend on this ratio.