A blood purification apparatus comprising: a blood circuit that includes an arterial blood circuit and a venous blood circuit, and that can extracorporeally circulate blood of a patient from a distal end of the arterial blood circuit to a distal end of the venous blood circuit; a blood purification device that purifies the blood flowing in the blood circuit by being interposed between the arterial blood circuit and the venous blood circuit of the blood circuit, that has a blood flow route through which the blood of the patient flows via a blood purification membrane for purifying the blood and a dialysate flow route through which a dialysate flows, and that has a blood inlet which can introduce the blood to the blood flow route, a blood outlet which can draw out the blood from the blood flow route, a dialysate inlet which can introduce the dialysate to the dialysate flow route, and a dialysate outlet which can discharge the dialysate from the dialysate flow route; a blood pump that is arranged in the arterial blood circuit, and that is capable of forward rotation which rotates in a direction of circulating the blood during blood purification treatment and reverse rotation which rotates in a direction opposite to the direction of circulating the blood during the blood purification treatment; a priming solution supply line (Lc, Ld, Le) that is connected to the blood circuit, and that can supply a priming solution into the blood circuit; a priming solution discharge unit that can cause the priming solution supplied by the priming solution supply line (Lc, Ld, Le) to overflow from the blood circuit so as to be discharged outward; a first valve device that can selectively close or open the priming solution supply line (Lc, Ld, Le); a second valve device that can selectively close or open the priming solution discharge unit; and a control device that can control the blood pump, the first valve device, and the second valve device, wherein the blood purification apparatus is suitable for performing priming in a state where the blood inlet of the blood purification device faces upward, characterized in that during the priming, the control device is configured to perform an air purging step of causing the blood pump to perform the reverse rotation, and opening the priming solution discharge unit and the priming solution supply line (Lc, Ld, Le) so as to discharge the priming solution from the priming solution discharge unit, while supplying the priming solution from the priming solution supply line (Lc, Ld, Le). a blood circuit that includes an arterial blood circuit and a venous blood circuit, and that can extracorporeally circulate blood of a patient from a distal end of the arterial blood circuit to a distal end of the venous blood circuit; a blood purification device that purifies the blood flowing in the blood circuit by being interposed between the arterial blood circuit and the venous blood circuit of the blood circuit, that has a blood flow route through which the blood of the patient flows via a blood purification membrane for purifying the blood and a dialysate flow route through which a dialysate flows, and that has a blood inlet which can introduce the blood to the blood flow route, a blood outlet which can draw out the blood from the blood flow route, a dialysate inlet which can introduce the dialysate to the dialysate flow route, and a dialysate outlet which can discharge the dialysate from the dialysate flow route; a blood pump that is arranged in the arterial blood circuit, and that is capable of forward rotation which rotates in a direction of circulating the blood during blood purification treatment and reverse rotation which rotates in a direction opposite to the direction of circulating the blood during the blood purification treatment; a priming solution supply line (Lc, Ld, Le) that is connected to the blood circuit, and that can supply a priming solution into the blood circuit; a priming solution discharge unit that can cause the priming solution supplied by the priming solution supply line (Lc, Ld, Le) to overflow from the blood circuit so as to be discharged outward; a first valve device that can selectively close or open the priming solution supply line (Lc, Ld, Le); a second valve device that can selectively close or open the priming solution discharge unit; and a control device that can control the blood pump, the first valve device, and the second valve device, wherein the blood purification apparatus is suitable for performing priming in a state where the blood inlet of the blood purification device faces upward, characterized in that during the priming, the control device is configured to perform an air purging step of causing the blood pump to perform the reverse rotation, and opening the priming solution discharge unit and the priming solution supply line (Lc, Ld, Le) so as to discharge the priming solution from the priming solution discharge unit, while supplying the priming solution from the priming solution supply line (Lc, Ld, Le).