A blood purification device allows confirming whether a specific peak is provided by a means, such as a ultrafiltration pump, to concentrate the blood or not and also allows measuring certainly and accurately a blood re-circulation with reduction of parameters which can provide a ratio of re-circulating blood. The blood purification device including blood circuit route 1 composed of arterial blood circuit route 1a and venous blood circuit route 1b, blood pump 3, dialyzer 2, water removal pump 8 which can provide the specific peak in the variation of blood concentration by removing water rapidly in a short period of time, and detecting means detecting said specific peak, can measure the blood re-circulation based on the specific peak detected by the detecting means. The re-circulating blood flowing is the blood which was introduced again to arterial blood circuit route 1a after it had been returned to a patient from venous blood circuit route 1b. The detecting means includes first detecting means 5a installed in arterial blood circuit route 1a and second detecting means 5b installed in venous blood circuit route 1b.
