A blood processing apparatus in which whether or not circuit components are connected is ascertained, and a method of ascertaining a failure in such circuit connection is provided. By closing a blood return valve, disposed in a blood return passage for returning a blood, processed in a first blood processing unit, to patients body, blocking a blood circuit including a blood introducing passage, the blood return passage and a delivery passage for delivering unnecessary blood component from the first blood processing unit, driving a first pump in the delivery passage in normal direction, and driving an air pump, fluidly connected with the blood introducing passage and the blood return passage, in reverse direction to discharge air from the circuit to generate negative pressure inside the circuit and the first blood processing unit, connection between the circuit and the first blood processing unit is ascertained.
