A blood purification apparatus has a blood circuit with an arterial blood circuit (1) and a venous blood circuit (2). Blood is extracorporeally circulated from a tip of the arterial blood circuit (1) to a tip of the venous blood circuit (2). A dialyzer (3) is interposed between the arterial blood circuit (1) and the venous blood circuit (2) to purify the blood. The blood purification apparatus can substitute the blood inside the blood circuit during returning of the blood after treatment. An air circulation line is connected to a predetermined portion in the blood circuit to circulate air. An air pump (18) supplies the air into the blood circuit, via the air circulation line. A control device (19) controls the air pump (18) to supply the air into the blood circuit during returning of the blood.
