A blood purification apparatus comprising a blood purification device including blood purification membranes that are capable of purifying blood; a blood circuit including an arterial blood circuit and a venous blood circuit that are connected to the blood purification device, the blood circuit allowing blood of a patient to extracorporeally circulate from a distal end of the arterial blood circuit to a distal end of the venous blood circuit; a dialysate introduction line connected to the blood purification device and that allows dialysate to be introduced into the blood purification device, a dialysate drain line connected to the blood purification device and that allows waste liquid to be drained from the blood purification device; and a blood pump provided to the arterial blood circuit and that is capable of causing the blood of the patient to extracorporeally circulate from the arterial blood circuit toward the venous blood circuit, the blood purification apparatus being capable of performing priming by supplying a priming solution into the blood circuit with an activation of the blood pump, wherein, when the priming solution is drained during priming, the priming solution supplied into the blood circuit is allowed to be drained through the dialysate drain line, characterized in that (i) the distal ends of the arterial blood circuit and the venous blood circuit are both connectable to one priming-solution storage bag that stores the priming solution and to be shared by the two blood circuits (1a, 1b), and in that, when the priming solution is drained during priming, the priming solution in the priming-solution storage bag is allowed to be supplied into the blood circuit, and/or (ii) the distal end of the arterial blood circuit is connectable to an arterial priming-solution storage bag that stores the priming solution while the distal end of the venous blood circuit is connectable to a venous priming-solution storage bag being comprised by the blood purification apparatus and that is configured to store the priming solution, and in that, when the priming solution is drained during priming, the priming solution in the arterial priming-solution storage bag or in the venous priming-solution storage bag is allowed to be supplied into the blood circuit. a blood purification device including blood purification membranes that are capable of purifying blood; a blood circuit including an arterial blood circuit and a venous blood circuit that are connected to the blood purification device, the blood circuit allowing blood of a patient to extracorporeally circulate from a distal end of the arterial blood circuit to a distal end of the venous blood circuit; a dialysate introduction line connected to the blood purification device and that allows dialysate to be introduced into the blood purification device, a dialysate drain line connected to the blood purification device and that allows waste liquid to be drained from the blood purification device; and a blood pump provided to the arterial blood circuit and that is capable of causing the blood of the patient to extracorporeally circulate from the arterial blood circuit toward the venous blood circuit, the blood purification apparatus being capable of performing priming by supplying a priming solution into the blood circuit with an activation of the blood pump, wherein, when the priming solution is drained during priming, the priming solution supplied into the blood circuit is allowed to be drained through the dialysate drain line, characterized in that (i) the distal ends of the arterial blood circuit and the venous blood circuit are both connectable to one priming-solution storage bag that stores the priming solution and to be shared by the two blood circuits (1a, 1b), and in that, when the priming solution is drained during priming, the priming solution in the priming-solution storage bag is allowed to be supplied into the blood circuit, and/or (ii) the distal end of the arterial blood circuit is connectable to an arterial priming-solution storage bag that stores the priming solution while the distal end of the venous blood circuit is connectable to a venous priming-solution storage bag being comprised by the blood purification apparatus and that is configured to store the priming solution, and in that, when the priming solution is drained during priming, the priming solution in the arterial priming-solution storage bag or in the venous priming-solution storage bag is allowed to be supplied into the blood circuit. (i) the distal ends of the arterial blood circuit and the venous blood circuit are both connectable to one priming-solution storage bag that stores the priming solution and to be shared by the two blood circuits (1a, 1b), and in that, when the priming solution is drained during priming, the priming solution in the priming-solution storage bag is allowed to be supplied into the blood circuit, and/or (ii) the distal end of the arterial blood circuit is connectable to an arterial priming-solution storage bag that stores the priming solution while the distal end of the venous blood circuit is connectable to a venous priming-solution storage bag being comprised by the blood purification apparatus and that is configured to store the priming solution, and in that, when the priming solution is drained during priming, the priming solution in the arterial priming-solution storage bag or in the venous priming-solution storage bag is allowed to be supplied into the blood circuit.