A blood purification apparatus comprising: a blood circuit for allowing blood of a patient to extracorporeally circulate, including an arterial blood circuit and a venous blood circuit; a blood purification device connected to the arterial blood circuit and to the venous blood circuit and being configured to purify the blood flowing through the blood circuit; a dialysate introduction line for introducing dialysate into the blood purification device; a dialysate drain line for draining waste liquid from the blood purification device; a liquid-delivering device capable of delivering liquid such that a volume of the dialysate to be introduced into the blood purification device and a volume of the waste liquid to be drained from the blood purification device are substantially equalized; and an ultrafiltration pump capable of removing, through the blood purification device, water from the blood flowing through the blood circuit, wherein the blood purification apparatus includes an ultrafiltration line (L3, L4) one end of which is connected to the dialysate introduction line at a position on a downstream side with respect to the liquid-delivering device, wherein the ultrafiltration line (L3, L4) is provided with the ultrafiltration pump, and wherein the ultrafiltration pump is capable of performing ultrafiltration by drawing the dialysate in the dialysate introduction line through the ultrafiltration line (L3, L4) such that the volume of the dialysate to be introduced into the blood purification device becomes smaller than the volume of the waste liquid to be drained from the blood purification device, wherein the liquid-delivering device and the ultrafiltration pump are each a pump capable of delivering liquid by periodically undergoing a suction step of taking in the liquid and a discharge step of discharging the taken liquid, and wherein the ultrafiltration pump is configured to operate only in a period where the liquid-delivering device is undergoing the discharge step, characterised in that the ultrafiltration pump is configured to operate on condition that a pressure of liquid discharge in the discharge step undergone by the liquid-delivering device exceeds a predetermined value. a blood circuit for allowing blood of a patient to extracorporeally circulate, including an arterial blood circuit and a venous blood circuit; a blood purification device connected to the arterial blood circuit and to the venous blood circuit and being configured to purify the blood flowing through the blood circuit; a dialysate introduction line for introducing dialysate into the blood purification device; a dialysate drain line for draining waste liquid from the blood purification device; a liquid-delivering device capable of delivering liquid such that a volume of the dialysate to be introduced into the blood purification device and a volume of the waste liquid to be drained from the blood purification device are substantially equalized; and an ultrafiltration pump capable of removing, through the blood purification device, water from the blood flowing through the blood circuit, wherein the blood purification apparatus includes an ultrafiltration line (L3, L4) one end of which is connected to the dialysate introduction line at a position on a downstream side with respect to the liquid-delivering device, wherein the ultrafiltration line (L3, L4) is provided with the ultrafiltration pump, and wherein the ultrafiltration pump is capable of performing ultrafiltration by drawing the dialysate in the dialysate introduction line through the ultrafiltration line (L3, L4) such that the volume of the dialysate to be introduced into the blood purification device becomes smaller than the volume of the waste liquid to be drained from the blood purification device, wherein the liquid-delivering device and the ultrafiltration pump are each a pump capable of delivering liquid by periodically undergoing a suction step of taking in the liquid and a discharge step of discharging the taken liquid, and wherein the ultrafiltration pump is configured to operate only in a period where the liquid-delivering device is undergoing the discharge step, characterised in that the ultrafiltration pump is configured to operate on condition that a pressure of liquid discharge in the discharge step undergone by the liquid-delivering device exceeds a predetermined value.