A blood purification apparatus comprising: a blood circuit for extracorporeally circulating the blood of a patient; a blood purification device that is connected to the blood circuit, and that purifies the extracorporeally circulating blood; a dialysate introduction line for introducing a dialysate into the blood purification device; a dialysate discharge line for discharging a discharge solution generated due to the blood purification performed by the blood purification device, from the blood purification device; a concentration detection device that can detect concentration of the discharge solution flowing in the dialysate discharge line; and an arithmetic device that can obtain a concentration change in the discharge solution, based on a first predetermined value being an initial value when blood purification treatment starts detected by the concentration detection device and a second predetermined value being a current measurement value detected after a predetermined period of time has elapsed therefrom, wherein the blood purification apparatus can perform real-time monitoring on a blood purification treatment state, based on the concentration change in the discharge solution, and the blood purification apparatus further comprises: a storage device that stores multiple treatment conditions, and a treatment condition change device that sets or changes the treatment conditions as requested, and that causes the concentration detection device to detect the initial values of the concentration of the discharge solution under each treatment condition; wherein the storage device stores the initial values corresponding to each treatment condition, and wherein when the treatment conditions are changed during blood purification treatment, the arithmetic device obtains the concentration change in the discharge solution, based on the initial value corresponding to the changed treatment conditions. a blood circuit for extracorporeally circulating the blood of a patient; a blood purification device that is connected to the blood circuit, and that purifies the extracorporeally circulating blood; a dialysate introduction line for introducing a dialysate into the blood purification device; a dialysate discharge line for discharging a discharge solution generated due to the blood purification performed by the blood purification device, from the blood purification device; a concentration detection device that can detect concentration of the discharge solution flowing in the dialysate discharge line; and an arithmetic device that can obtain a concentration change in the discharge solution, based on a first predetermined value being an initial value when blood purification treatment starts detected by the concentration detection device and a second predetermined value being a current measurement value detected after a predetermined period of time has elapsed therefrom, wherein the blood purification apparatus can perform real-time monitoring on a blood purification treatment state, based on the concentration change in the discharge solution, and the blood purification apparatus further comprises: a storage device that stores multiple treatment conditions, and a treatment condition change device that sets or changes the treatment conditions as requested, and that causes the concentration detection device to detect the initial values of the concentration of the discharge solution under each treatment condition; wherein the storage device stores the initial values corresponding to each treatment condition, and wherein when the treatment conditions are changed during blood purification treatment, the arithmetic device obtains the concentration change in the discharge solution, based on the initial value corresponding to the changed treatment conditions. a storage device that stores multiple treatment conditions, and a treatment condition change device that sets or changes the treatment conditions as requested, and that causes the concentration detection device to detect the initial values of the concentration of the discharge solution under each treatment condition; wherein the storage device stores the initial values corresponding to each treatment condition, and wherein when the treatment conditions are changed during blood purification treatment, the arithmetic device obtains the concentration change in the discharge solution, based on the initial value corresponding to the changed treatment conditions.