To provide a blood purification apparatus in which a lack of substitution fluid in an air-trap chamber can be recognized quickly and correctly. A blood purification apparatus includes a blood circuit 1, a dialyzer 2 capable of purifying the blood flowing through the blood circuit 1, a blood pump P1 provided to an arterial blood circuit 1a and that delivers the blood in the blood circuit, and an air-trap chamber 5 capable of collecting air in the blood flowing through the blood circuit 1. A peristaltic pump P3 (a substitution-fluid-infusion device) that is capable of infusing a substitution fluid is connected to the air-trap chamber 5. A substitution fluid layer is formable on a blood layer in the air-trap chamber 5. The air-trap chamber 5 is provided with a blood-interface-detecting device 6 that is capable of detecting an interface between the blood layer and the substitution fluid layer that are formed in the air-trap chamber 5. A lack of substitution fluid in the air-trap chamber 5 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 6.
