Device for haemodiafiltration, which comprises a first circuit for a dialysis solution and a second circuit for blood, so that the toxic substances from the blood flow pass into the dialysis liquid within a haemofilter (11), a liquid pump (20) located upstream of the haemofilter (11) in the first dialysis liquid circuit, is adapted for injecting replacement liquid into the dialysis solution flow after a blood pump (15) finishes pumping blood to the interior of the haemofilter (11).