An online blood purification system comprising: a blood purifier that comprises a first space and second space partitioned by a blood purification membrane and purifies blood by passing extracorporeal blood through the first space, a blood inlet side flow path through which the extracorporeal blood flows prior to flowing into the first space, a blood outlet side flow path through which the extracorporeal blood flows after having flown out from the first space, a fluid supply flow path for supplying a fluid to the second space, and a fluid recovery flow path for recovering fluid from the second space, characterized in that the online blood purification system further comprises: an arterial side replacement fluid supply flow path that branches from the fluid supply flow path and is connected to the blood inlet side flow path, and a venous side replacement fluid supply flow path that branches from the fluid supply flow path and is connected to the blood outlet side flow path; wherein the arterial side replacement fluid supply flow path and the venous side replacement fluid supply flow path respectively comprise a fluid transfer means (51 and 52) that can be independently controlled, wherein the blood inlet side flow path and the blood outlet side flow path are interconnectable, and wherein residual blood remaining in the first space can be recovered by closing either the arterial side replacement fluid supply flow path or the venous side replacement fluid supply flow path and allowing fluid for recovering blood to flow into the bllod inlet side flow path or the blood outlet side flow path from the other replacement fluid supply flow path that is not closed. a blood purifier that comprises a first space and second space partitioned by a blood purification membrane and purifies blood by passing extracorporeal blood through the first space, a blood inlet side flow path through which the extracorporeal blood flows prior to flowing into the first space, a blood outlet side flow path through which the extracorporeal blood flows after having flown out from the first space, a fluid supply flow path for supplying a fluid to the second space, and a fluid recovery flow path for recovering fluid from the second space, characterized in that the online blood purification system further comprises: an arterial side replacement fluid supply flow path that branches from the fluid supply flow path and is connected to the blood inlet side flow path, and a venous side replacement fluid supply flow path that branches from the fluid supply flow path and is connected to the blood outlet side flow path; wherein the arterial side replacement fluid supply flow path and the venous side replacement fluid supply flow path respectively comprise a fluid transfer means (51 and 52) that can be independently controlled, wherein the blood inlet side flow path and the blood outlet side flow path are interconnectable, and wherein residual blood remaining in the first space can be recovered by closing either the arterial side replacement fluid supply flow path or the venous side replacement fluid supply flow path and allowing fluid for recovering blood to flow into the bllod inlet side flow path or the blood outlet side flow path from the other replacement fluid supply flow path that is not closed. an arterial side replacement fluid supply flow path that branches from the fluid supply flow path and is connected to the blood inlet side flow path, and a venous side replacement fluid supply flow path that branches from the fluid supply flow path and is connected to the blood outlet side flow path; wherein the arterial side replacement fluid supply flow path and the venous side replacement fluid supply flow path respectively comprise a fluid transfer means (51 and 52) that can be independently controlled, wherein the blood inlet side flow path and the blood outlet side flow path are interconnectable, and wherein residual blood remaining in the first space can be recovered by closing either the arterial side replacement fluid supply flow path or the venous side replacement fluid supply flow path and allowing fluid for recovering blood to flow into the bllod inlet side flow path or the blood outlet side flow path from the other replacement fluid supply flow path that is not closed.