A blood purification device, comprising: a blood circuit for extracorporeally circulating blood of a patient; a liquid supply line for supplying a supply liquid to a blood purifier provided on the blood circuit or to the blood circuit; a peristaltic liquid supply pump (24, 64, 74) provided on the blood circuit or on the liquid supply line; a peristaltic waste liquid pump provided on a waste liquid line through which a waste liquid is discharged from the blood purifier; a water balance amount detection unit for detecting a water balance amount of the supply liquid and the waste liquid; and a water balance amount control unit that corrects one or both of pump speeds of the liquid supply pump (24, 64, 74) and the waste liquid pump so that the water balance amount detected by the water balance amount detection unit matches a target water balance amount, characterized in that the water balance amount control unit is configured to switch between a first correction condition for correcting the pump speed on every water balance amount detection and a second correction condition for correcting the pump speed based on a plurality of water balance amount detection results, wherein the water balance amount control unit performs pump speed correction under the first correction condition at the beginning of a treatment and switches to pump speed correction under the second correction condition when a stability determination condition for determining that the water balance amount is stabilized is satisfied, and wherein the pump speed correction under the second correction condition is performed using a middle value of the plurality of water balance amount detection results. a blood circuit for extracorporeally circulating blood of a patient; a liquid supply line for supplying a supply liquid to a blood purifier provided on the blood circuit or to the blood circuit; a peristaltic liquid supply pump (24, 64, 74) provided on the blood circuit or on the liquid supply line; a peristaltic waste liquid pump provided on a waste liquid line through which a waste liquid is discharged from the blood purifier; a water balance amount detection unit for detecting a water balance amount of the supply liquid and the waste liquid; and a water balance amount control unit that corrects one or both of pump speeds of the liquid supply pump (24, 64, 74) and the waste liquid pump so that the water balance amount detected by the water balance amount detection unit matches a target water balance amount, characterized in that the water balance amount control unit is configured to switch between a first correction condition for correcting the pump speed on every water balance amount detection and a second correction condition for correcting the pump speed based on a plurality of water balance amount detection results, wherein the water balance amount control unit performs pump speed correction under the first correction condition at the beginning of a treatment and switches to pump speed correction under the second correction condition when a stability determination condition for determining that the water balance amount is stabilized is satisfied, and wherein the pump speed correction under the second correction condition is performed using a middle value of the plurality of water balance amount detection results.