To provide a blood purification apparatus that is capable of, with no preparatory operations, performing substitution by immediately supplying dialysate in a dialysate introduction line to a blood circuit during ultrafiltration treatment, or performing blood return by immediately supplying the dialysate in the dialysate introduction line to the blood circuit after the ultrafiltration treatment. A blood purification apparatus includes a dialyzer 1 that purifies blood flowing in a blood circuit, a dialysate introduction line L1 through which dialysate is allowed to be introduced into the dialyzer 1, a dialysate drain line L2 through which drain liquid from the dialyzer 1 is allowed to be drained, and an ultrafiltration pump 10 capable of removing water from the blood flowing in the blood circuit. The blood purification apparatus is capable of performing substitution or blood return by supplying the dialysate in the dialysate introduction line L1 to the blood circuit. In an ultrafiltration treatment in which the ultrafiltration pump 10 is activated for ultrafiltration while the introduction of the dialysate into the dialyzer 1 is stopped, dialysate delivery is performed while the introduction of the dialysate into the dialyzer 1 is prevented.
