A blood purification apparatus that can make the specific peak to be imparted to the blood more distinct, and therefore can detect blood recirculation reliably and precisely, is presented. It contains a blood circuit, a blood pump, a dialyzer, dialysate introduction and discharge lines, a duplex pump, a pressurizing pump, a blood concentration means, and detection means that detect the specific peak imparted by the blood concentration means. The blood concentration means is configured to have an atmosphere release line, the tip of which is open to the atmosphere, that extends from between the pressurizing pump and the duplex pump in the dialysate discharge line, and to have an electromagnetic valve that can open or close the atmosphere release line. The electromagnetic valve opens the atmosphere release line to impart the specific peak by rapidly concentrating the blood flowing in the dialyzer for a short period of time.
