The invention concerns a filter for thermoplastic material, with a housing (I) in which a support element (4), in the shape of a piston, for two nests of screens (8, 9) is mounted for longitudinal motion along its axis. In the filtration position, the melt flows out of a feed channel (2) and through inlet channels (10-13) into distributor spaces (14, 15) located in front of the nests of screens (8 or 9), then through the screens and through outlet channels (18, 19) into a discharge channel (3). Each distributor space (14, 15) is sub-divided into sections by at least one wall (21), one inlet channel (10-13) opening out into each such section. By displacing the support element (4) longitudinally, it can be brought into different back-flushing positions for each nest of screens (8 or 9). In these positions, all the inlet channels, except one which is associated with the nest of sieves being back-flushed, are closed, the one open inlet channel acting as the removal channel for the flushing melt and the impurities it carries with it. The flushing melt is fed, through a flushing channel (23) which runs entirely within the support element (4), to the collector space (16 or 17) behind the nest of screens (8 or 9) being flushed.