A blood purification apparatus comprising: a blood circuit that includes an arterial blood circuit and a venous blood circuit, and that extracorporeally circulates 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 is interposed between the arterial blood circuit and the venous blood circuit in the blood circuit, and that is configured to purify the blood flowing in the blood circuit; a blood pump that is arranged in the arterial blood circuit; a venous air trap chamber that is connected to the venous blood circuit; a liquid level adjustment device that is connected to the venous air trap chamber, and that is configured to optionally introduce or discharge air into or from an upper portion of the venous air trap chamber; a control device that is configured to adjust a liquid level formed inside the venous air trap chamber to have the height at any desired position by operating the liquid level adjustment device; and a priming solution supplying line that is configured to supply a priming solution to the arterial blood circuit and the venous blood circuit during priming, wherein during the priming, the control device is configured to fill the arterial blood circuit and the venous blood circuit with the priming solution supplied from the priming solution supplying line by operating the liquid level adjustment device at any desired timing, wherein the blood purification apparatus further comprises a dialysate introduction line that introduces a dialysate to the blood purification device; and a dialysate discharge line that discharges the dialysate from the blood purification device, characterized in that the distal end of the arterial blood circuit and the distal end of the venous blood circuit are respectively configured to be connected to the dialysate discharge line or a connection line which connects the dialysate introduction line and the dialysate discharge line to each other during the priming, and wherein the distal end of the arterial blood circuit and the distal end of the venous blood circuit are configured to discharge the priming solution supplied from the priming solution supplying line via the dialysate discharge line. a blood circuit that includes an arterial blood circuit and a venous blood circuit, and that extracorporeally circulates 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 is interposed between the arterial blood circuit and the venous blood circuit in the blood circuit, and that is configured to purify the blood flowing in the blood circuit; a blood pump that is arranged in the arterial blood circuit; a venous air trap chamber that is connected to the venous blood circuit; a liquid level adjustment device that is connected to the venous air trap chamber, and that is configured to optionally introduce or discharge air into or from an upper portion of the venous air trap chamber; a control device that is configured to adjust a liquid level formed inside the venous air trap chamber to have the height at any desired position by operating the liquid level adjustment device; and a priming solution supplying line that is configured to supply a priming solution to the arterial blood circuit and the venous blood circuit during priming, wherein during the priming, the control device is configured to fill the arterial blood circuit and the venous blood circuit with the priming solution supplied from the priming solution supplying line by operating the liquid level adjustment device at any desired timing, wherein the blood purification apparatus further comprises a dialysate introduction line that introduces a dialysate to the blood purification device; and a dialysate discharge line that discharges the dialysate from the blood purification device, characterized in that the distal end of the arterial blood circuit and the distal end of the venous blood circuit are respectively configured to be connected to the dialysate discharge line or a connection line which connects the dialysate introduction line and the dialysate discharge line to each other during the priming, and wherein the distal end of the arterial blood circuit and the distal end of the venous blood circuit are configured to discharge the priming solution supplied from the priming solution supplying line via the dialysate discharge line.