A blood purification system comprising: (a) a blood treatment device which comprises: a blood treatment circuit which comprises a blood purification unit, configured to supply blood to the blood purification unit, and to recover purified blood from the blood purification unit; a first supply pump; a first reservoir; a therapeutic fluid supply circuit configured to supply a therapeutic fluid used in the blood treatment circuit that is stored in the first reservoir to the blood treatment circuit by the first supply pump; and a first weight scale configured to measure the weight of the first reservoir; characterized in that the blood purification system further comprises (b) a therapeutic fluid supply device comprising: a second supply pump; a second reservoir; a therapeutic fluid refill circuit configured to refill therapeutic fluid stored in the second reservoir into the first reservoir at a predetermined flow rate by a second supply pump; and a second weight scale configured to measure the weight of the second reservoir; and (c) a controller configured to calculate the actual flow rate of the first supply pump from the variation in the sum of the weight of the first reservoir measured by the first weight scale and the weight of the second reservoir measured by the second weight scale and to control the first supply pump based on the actual flow rate. (a) a blood treatment device which comprises: a blood treatment circuit which comprises a blood purification unit, configured to supply blood to the blood purification unit, and to recover purified blood from the blood purification unit; a first supply pump; a first reservoir; a therapeutic fluid supply circuit configured to supply a therapeutic fluid used in the blood treatment circuit that is stored in the first reservoir to the blood treatment circuit by the first supply pump; and a first weight scale configured to measure the weight of the first reservoir; characterized in that the blood purification system further comprises a blood treatment circuit which comprises a blood purification unit, configured to supply blood to the blood purification unit, and to recover purified blood from the blood purification unit; a first supply pump; a first reservoir; a therapeutic fluid supply circuit configured to supply a therapeutic fluid used in the blood treatment circuit that is stored in the first reservoir to the blood treatment circuit by the first supply pump; and a first weight scale configured to measure the weight of the first reservoir; characterized in that the blood purification system further comprises (b) a therapeutic fluid supply device comprising: a second supply pump; a second reservoir; a therapeutic fluid refill circuit configured to refill therapeutic fluid stored in the second reservoir into the first reservoir at a predetermined flow rate by a second supply pump; and a second weight scale configured to measure the weight of the second reservoir; and a second supply pump; a second reservoir; a therapeutic fluid refill circuit configured to refill therapeutic fluid stored in the second reservoir into the first reservoir at a predetermined flow rate by a second supply pump; and a second weight scale configured to measure the weight of the second reservoir; and (c) a controller configured to calculate the actual flow rate of the first supply pump from the variation in the sum of the weight of the first reservoir measured by the first weight scale and the weight of the second reservoir measured by the second weight scale and to control the first supply pump based on the actual flow rate.