It is intended to provide a blood dialysis apparatus by which bubbles can be almost completely removed from the circuit in the blood dialysis apparatus, the invasion of bubbles into the circuit can be prevented at blood removal, and the completion of the blood return can be surely determined. A blood dialysis apparatus comprising an artery side blood circuit having a blood pump which is connected to the artery side of a dialyzer, a vein side blood circuit having a vein chamber which is connected to the vein side of the dialyzer, and a dialysate-feeding line and a dialysate-discharging line each connected to the side face of the dialyzer, wherein a cramp in an overflow line that is provided in the vein chamber in the vein side blood circuit is switched on/off, bubbles are removed by the liquid-feeding movement of the blood pump and, moreover, bubbles remaining in the blood circuits are completely removed by the pulsation movement of the blood pump in the step of automatic priming.
