A blood purification device that accurately finds the colloid osmotic pressure (CP) of the blood of a patient. According to the present invention, a blood purification device has a dialyzer (3) in which a blood channel and a dialysate channel are separated by a hollow fiber membrane (3e). A water removal pump (8) pulls water from blood that is in the blood channel through the hollow fiber membrane (3e) and into the dialysate channel. Four detection parts (P1–P4) measure the pressure of liquid that flows into the blood channel, liquid that has flowed out from the blood channel, dialysate that flows into the dialysate channel, and dialysate that has flowed out from the dialysate channel. A transmembrane pressure difference (TMP) can be calculated on the basis of the four pressures. The water removal pump (8) is stopped. The blood channel is filled with a priming liquid, and the transmembrane pressure difference (TMPa) is measured. The blood channel is then filled with the blood of a patient, and the transmembrane pressure difference (TMPb) is measured. The colloid osmotic pressure (CP) of the blood of the patient can be calculated on the basis of TMPa and TMPb. Plasma total protein (TP) is found from the colloid osmotic pressure (CP). The nutritional state of the patient can be estimated on the basis of the plasma total protein (TP).L'invention concerne un dispositif de purification du sang qui trouve avec précision la pression osmotique colloïdale (CP) du sang d'un patient. Selon la présente invention, un dispositif de purification de sang comporte un dialyseur (3) dans lequel un canal de sang et un canal de dialysat sont séparés par une membrane à fibres creuses (3e). Une pompe d'élimination d'eau (8) aspire l'eau du sang qui se trouve dans le canal de sang à travers la membrane à fibres creuses (3e) et dans le canal de dialysat. Quatre parties de détection (P1 à P4) mesurent la pression de liquide qui s'écoule dans le canal de sang, de liquide qui s'est écoulé à pa
