To provide a blood purification apparatus that does not require a priming-solution-collecting bag to be connected to the apparatus only when priming is performed, so that the cost for performing priming can be reduced. In a blood purification apparatus capable of performing priming by supplying a priming solution into a blood circuit 1 with an activation of a blood pump P1, when the priming solution is drained during priming, the priming solution supplied into the blood circuit 1 is allowed to be drained through a dialysate drain line. Furthermore, the priming solution supplied into the blood circuit 1 is allowed to flow into the dialysate drain line L2 through a substitution line L3 and to be drained.
