Provided is a hemodialyzer for preventing beforehand any medical accident such as air inclusion or blood loss during a medical treatment by detecting even a minute connection failure, if any. The hemodialyzer includes a hemodialyzer (D), a dialysate supply line (L1), a dialysate discharge line (L2), an artery side blood line (L3), a vein side blood line (L4), first fluid feeding means (P1), second fluid feeding means (P2), third fluid feeding means (P3), a blood pump (P4), a vein side chamber (C V ), an overflow line (L5), a venous pressure monitor line (L6), dialysate pressure measuring means (M T ), venous pressure measuring means (M V ), clamps (CL L4 , CL L5 ), control means (G1) that closes the clamp (CL L5 ) when a predetermined given period of time has elapsed from the start of priming, and operates the third fluid feeding means (P3) in a reverse filtration direction or in a fluid removal direction, and then opens the clamp (CL L5 ) when a predetermined given period of time has elapsed, and judging means (J1) that compares a pressure variation amount in the dialysate pressure or the venous pressure for a period of time when the clamp (CL L5 ) is closed with a predetermined reference value to judge normality of a circuit connection.
