We have conducted a photometric survey of the globular cluster NGC 6397 in a search for variable stars. We obtained ~11h of time-resolved photometric images with one European Southern Observatory-Very Large Telescope using the FOcal Reducer and low dispersion Spectrograph imager distributed over two consecutive nights. We analysed 8391 light curves of stars brighter than magnitude 23 with the 465-nm filter, and we identified 412 variable stars, reaching ~4.8+/-0.2 per cent of variability with time-scales between 0.004 and 2d, with amplitudes variation greater than +/-0.2mag.