A blood purification apparatus comprising: a blood circuit including an arterial blood circuit and a venous blood circuit and being configured such that blood of a patient can extracorporeally circulate through the blood circuit; a blood purification device provided between the arterial blood circuit and the venous blood circuit and that is configured to purify the blood flowing in the blood circuit; a dialysate introduction line configured such that dialysate can be introduced through the dialysate introduction line into the blood purification device; a dialysate drain line configured such that drain liquid from the blood purification device can be drained through the dialysate drain line; and an ultrafiltration pump configured to remove water from the blood flowing in the blood circuit through the blood purification device, wherein the blood purification apparatus is configured to perform substitution or blood return by supplying the dialysate in the dialysate introduction line to the blood circuit, and wherein, in an ultrafiltration treatment in which the ultrafiltration pump is activated for ultrafiltration while the introduction of the dialysate into the blood purification device is stopped, the blood purification apparatus is configured to perform dialysate delivery while the introduction of the dialysate into the blood purification device is prevented, the blood purification apparatus further comprising: a fluid delivering device configured to deliver the dialysate in the dialysate introduction line to the blood purification device; a bypass line (L3, L4) configured such that the dialysate in the dialysate introduction line can bypass the blood purification device and flow into the dialysate drain line; and a closing device configured to close a flow route of the dialysate introduction line that extends between a connection to the bypass line (L3, L4) and the blood purification device, wherein the blood purification apparatus is configured such that, during the ultrafiltration treatment, the flow route is closed by the closing device, and the dialysate that is delivered by the fluid delivering device flows into the bypass line (L3, L4), characterized in that the blood purification apparatus is configured such that, when the ultrafiltration treatment is started, the fluid delivering device is stopped so that the flow of the dialysate in the dialysate introduction line is stopped and then, during the ultrafiltration treatment, the fluid delivering device is activated and the dialysate delivery is performed at a preset timing. a blood circuit including an arterial blood circuit and a venous blood circuit and being configured such that blood of a patient can extracorporeally circulate through the blood circuit; a blood purification device provided between the arterial blood circuit and the venous blood circuit and that is configured to purify the blood flowing in the blood circuit; a dialysate introduction line configured such that dialysate can be introduced through the dialysate introduction line into the blood purification device; a dialysate drain line configured such that drain liquid from the blood purification device can be drained through the dialysate drain line; and an ultrafiltration pump configured to remove water from the blood flowing in the blood circuit through the blood purification device, wherein the blood purification apparatus is configured to perform substitution or blood return by supplying the dialysate in the dialysate introduction line to the blood circuit, and wherein, in an ultrafiltration treatment in which the ultrafiltration pump is activated for ultrafiltration while the introduction of the dialysate into the blood purification device is stopped, the blood purification apparatus is configured to perform dialysate delivery while the introduction of the dialysate into the blood purification device is prevented, the blood purification apparatus further comprising: a fluid delivering device configured to deliver the dialysate in the dialysate introduction line to the blood purification device; a bypass line (L3, L4) configured such that the dialysate in the dialysate introduction line can bypass the blood purification device and flow into the dialysate drain line; and a closing device configured to close a flow route of the dialysate introduction line that extends between a connection to the bypass line (L3, L4) and the blood purification device, wherein the blood purification apparatus is configured such that, during the ultrafiltration treatment, the flow route is closed by the closing device, and the dialysate that is delivered by the fluid delivering device flows into the bypass line (L3, L4), characterized in that the blood purification apparatus is configured such that, when the ultrafiltration treatment is started, the fluid delivering device is stopped so that the flow of the dialysate in the dialysate introduction line is stopped and then, during the ultrafiltration treatment, the fluid delivering device is activated and the dialysate delivery is performed at a preset timing.