A blood purification apparatus capable of performing pre-substitution and post-substitution simultaneously is provided. A patient's blood flows through an arterial blood circuit and reaches a dialyzer. The blood purified by the dialyzer flows through a venous blood circuit and returns into the patient. A first substitution pump delivers substitution fluid through a substitution line to a branching point. A pre-substitution line connects the branching point and the arterial blood circuit to each other and is provided with a second substitution pump at a halfway position thereof. A post-substitution line connects the branching point and the venous blood circuit to each other and is provided with a check valve at a halfway position thereof. When the two pumps are activated simultaneously, the substitution fluid is supplied to the arterial blood circuit (pre-substitution). When only the first substitution pump is activated, the substitution fluid is supplied to the venous blood circuit (post-substitution). When the flow rate of the second substitution pump is set higher than the flow rate of the first substitution pump, pre-substitution and post-substitution are performed simultaneously.