A blood purification device whereby pre- and post-fluid replacement can be performed simultaneously. Blood of a patient passes through an artery-side blood circuit (1a) and reaches a dialyzer (2). Blood purified in the dialyzer (2) returns to the patient through a vein-side blood circuit (1b). A first fluid replacement pump (P3) sends a replenishing liquid to a branch point through a fluid replacement line (L3b). A pre-fluid replacement line (L3c) connects the branch point and the artery-side blood circuit (1a), and a second fluid replacement pump (P2) is present partway along the fluid replacement line (L3c). A post-fluid replacement line (L3d) connects the branch point and the vein-side blood circuit (1b), and a check valve (V1) is present partway along the post-fluid replacement line (L3d). Both pumps (P2, P3) are driven simultaneously, and the replenishing liquid is thereby supplied to the artery-side blood circuit (1a) (pre-fluid replacement). By driving of only the first fluid replacement pump (P3), the replenishing liquid is supplied to the vein-side blood circuit (1b) (post-fluid replacement). Pre-fluid replacement and post-fluid replacement are performed simultaneously by making the flow rate of the second fluid replacement pump (P2) greater than the flow rate of the first fluid replacement pump (P3).La présente invention concerne un dispositif de purification du sang permettant d'effectuer simultanément le remplacement pré et post-liquide. Le sang d'un·e patient·e passe à travers un circuit de sang côté artère (1a) et atteint un dialyseur (2). Le sang purifié dans le dialyseur (2) revient au·à la patient·e à travers un circuit de sang côté veine (1b). Une première pompe de remplacement de liquide (P3) envoie un liquide de réapprovisionnement à un point de ramification à travers une conduite de remplacement de liquide (L3b). Une conduite pré-remplacement de liquide (L3c) relie le point de ramification et le circuit de sang côté artère (1a), et une seconde p
