PROBLEM TO BE SOLVED: To provide a blood purifier in which the connection confirmation of a connecting line to a sampling opening can be automatically carried out prior to a priming step and the connection confirmation can be reliably and precisely carried out.SOLUTION: The blood purifier has a closed circuit forming means (solenoid valves V1 to V12) capable of forming a closed circuit in a flow channel including a predetermined site where a sampling opening 11 is formed, a duplex type pump 7 capable of pressurizing the inside of the closed circuit formed by the closed circuit forming means, a liquid pressure measuring means 12 for detecting the liquid pressure rise in the inside of the close circuit formed by the closed circuit forming means, and a control means 16 forming the closed circuit by the closed circuit forming means, pressurizing the inside of the closed circuit by the duplex type pump 7, thereafter causing the liquid pressure measuring means 12 to detect the liquid pressure rise in the closed circuit, thereby carrying out the connection confirmation of the connection line L8 to the sampling opening 11, prior to the priming step for filling the artery-side blood circuit 2 and a vein-side blood circuit 3 with dialysate.
