[Object] Disclosed is a blood purification apparatus that is capable of transitioning from priming to blood purification treatment while a fluid infusing line is still connected and is capable of improving workability during the transition from priming to blood purification treatment. Also disclosed is a priming method thereof.[Solution] During priming, the other end of the fluid infusing line L3 is connected at the same location as the connected location of the fluid infusing line L3, which is determined in accordance with whether either pre-fluid infusion in which replenishment fluid is supplied to an arterial air trap chamber 5 or post-fluid infusion in which the replenishment fluid is supplied to a venous air trap chamber 6 will be performed in the blood purification treatment process, and the end of an arterial blood circuit 2 and the end of a venous blood circuit 3 are connected together to communicate fluid. In addition, a blood pump 4 is driven in normal rotation or in reverse rotation while the replenishment fluid is supplied from the fluid infusing line L3, thereby discharging the replenishment fluid from an overflow line 6a.
