Provided is a blood purification apparatus which can continue to accurately monitor a blood purification treatment state even if treatment conditions are changed during blood purification treatment. The blood purification apparatus can perform real-time monitoring on the blood purification treatment state, based on a concentration change in a discharge solution which is obtained by an arithmetic device (12). The blood purification apparatus includes a treatment condition change device (13) that sets or changes treatment conditions as requested, and that causes a discharge solution concentration sensor (5) to previously detect concentration of the discharge solution under the respective treatment conditions, and a storage device (14) that stores the concentration of the discharge solution under the respective treatment conditions which is detected by the discharge solution concentration sensor (5) in the treatment condition change device (13), as a first predetermined value corresponding to the respective treatment conditions. When the treatment conditions are changed during blood purification treatment, the arithmetic device (12) obtains the concentration change in the discharge solution, based on the first predetermined value corresponding to the changed treatment conditions among the first predetermined values stored in the storage device (14).
