A blood purification apparatus includes a blood circuit, a dialyzer capable of purifying the blood flowing through the blood circuit, a blood pump provided to an arterial blood circuit and that delivers the blood in the blood circuit, and an air-trap chamber capable of collecting air in the blood flowing through the blood circuit. A peristaltic pump (a substitution-fluid-infusion device) that is capable of infusing a substitution fluid is connected to the air-trap chamber. A substitution fluid layer is formable on a blood layer in the air-trap chamber. The air-trap chamber is provided with a blood-interface-detecting device that is capable of detecting an interface between the blood layer and the substitution fluid layer that are formed in the air-trap chamber. A lack of substitution fluid in the air-trap chamber is detectable on the basis of the interface between the blood layer and the substitution fluid layer that is detected by the blood-interface-detecting device.
