[PROBLEMS] To provide a method of priming a blood circuit whereby the labor of a worker in the course of priming can be relieved while elevating the operation efficiency to thereby inhibit human error, the extension of priming time can be avoided, and the discharge of a priming solution containing an anticoagulant can be avoided. [MEANS FOR SOLVING PROBLEMS] In a blood purification unit comprising an arterial side blood circuit (1) and a vein side blood circuit (2) and having another blood circuit for the extracorporeal circulation of the patients blood, a dialyzer (3), which is provided between the arterial side blood circuit (1) and a vein side blood circuit (2) and purifies the blood flowing in the blood circuits, and an anticoagulant-injection line (L4) from which an anticoagulant can be injected into the blood circuits, wherein the end section of the arterial side blood circuit (1) and the end section of the vein side blood circuit (2) are connected together to form a circulation circuit and a blood pump (4) is driven so that the anticoagulant is injected from the anticoagulant-injection line (L4) while circulating a priming solution in the circulation circuit.
