A blood purification apparatus has a blood circuit, a blood pump and a blood purification instrument. A dialysate is introduced into and exits the blood purifying instrument. The blood purification apparatus has a pressure varying mechanism to vary pressure in a closed circuit under a condition where the blood circuit is formed as a closed circuit in a sealed condition connecting the tip end of the arterial blood circuit and the tip end of the venous blood circuit. A liquid leakage detecting mechanism detects liquid leakage in the blood circuit in accordance with pressure variation generated by the pressure varying mechanism.
