To provide a blood circuit by which a thrombus in blood flowing through an artery-side blood circuit in dialysis treatment can be separated in the blood circuit wherein the dialysis treatment and blood recovery are performed on-line or wherein a drip chamber is not connected with the artery- side blood circuit so as to reduce a blood quantity to be circulated extracorporeally.

The blood circuit is connected with a dialyzer 2 including a blood flowing path 2a and a hollow fiber membrane 3 for purifying the blood flowing through the path 2a and circulates the blood in the body. An artery-side blood circuit 1a is connected to the introduction port 2c of the blood flowing path for the dialyzer 2 and has a blood pump 5 arranged in the middle of the circuit. A vein-side blood circuit 1b is connected to the exit port 2d of the path 2a for the dialyzer 2 and has the drip chamber 7 connected in the middle of the circuit. A chamber 11 to be filled with the blood is connected to the artery-side blood circuit 1a and a filter net 12 is formed in the chamber 11.