PROBLEM TO BE SOLVED: To provide a blood purifying apparatus constituted so as to automatically decide the secretion of a vasodilator substance in an internal organ periodically high in metabolic rate from the change in blood pressure accompanied by infusion during dialysis and adjusting a water removing speed according to the decision result to prevent the lowering of blood pressure.SOLUTION: The blood purifying apparatus includes a blood purifier 1, a blood supply circuit 21, a blood pump 3, a blood return circuit 22, a dialyzing fluid supply flow channel 41, a dialyzing fluid pump 43, a dialyzing fluid discharge flow channel 42, water removing means 44, the fluid replenishing line 5 provided in the dialyzing fluid supply circuit 41 and the blood supply circuit 21 on the upstream side of the blood pump 3, the automatic on-off valves V1 and V2 provided in the blood supply circuit 21, the pressure measuring line 25 branched from the blood supply circuit 21 on the further upstream side of the automatic on-off valve V1, the pressure measuring means 24 provided at an end opposite to the blood supply circuit 21 of the pressure measuring line 25, the pressure change operation means 71 electrically connected to the pressure measuring means 24 and water removing control means 72 of the pressure change operation means 71 and water removing means 44.
