Provided are a blood purifying device and a method of calculating the flow rate in an access blood vessel using said blood purifying device, with which it is possible to obtain the flow rate in the access blood vessel accurately and in a short time, irrespective of the flow rate in the access blood vessel. This blood purifying device is provided with: a control means 11 for causing a blood pump 3 to be driven in the opposite direction to the direction during blood purification treatment, in a state in which an upstream side of an access blood vessel has been punctured using an arterial puncture needle, and a downstream side of the access blood vessel has been punctured using a venous puncture needle, thereby making it possible for blood to which a specific change has been imparted using a indicator-change imparting means to flow through the access blood vessel； and a calculating means 12 capable of calculating the flow rate in the access blood vessel on the basis of the flow rate when the blood pump 3 is driven in said opposite direction, the specific change imparted using the indicator-change imparting means, and a specific change detected by a detecting means E1 after the blood has flowed through the access blood vessel.
