An object of the present invention is to provide a blood purification apparatus and its priming method which can simply and easily automate the priming operation and also can surely and smoothly perform the bubble purging of the blood purification means (dialyzer). According to the blood purification apparatus of the present invention, it is characterized in that during the priming operation the control means (16) can sequentially control following steps under a condition in which the blood inlet port (3a) of the blood purification means (3) is set at a top position: an overflowing step in which the priming solution is supplied to the drip chamber (5) from the priming solution supplying line (Lc) via the venous blood circuit (2) and then discharged through the over flow line (6) by stopping the blood pump (4) and opening the valve means (V6)； a circulating step in which the priming solution is supplied to the arterial blood circuit (1) from the priming solution supplying line (Lc) via the venous blood circuit (2) and the blood purification means (3) by driving the blood pump (4) in reverse and closing the valve means (V6)； and a shifting step for shifting the circulating step to the overflowing step so long as the venous bubble detecting means (10) detects bubbles.
