A blood vessel anastomosing auxiliary tool consisting of a flexible hollow conduit having opening ends at the opposite ends thereof, and comprising, on the outer surface of the conduit, at least two bulky portions, a retrieving means for retrieving the conduit from within a blood vessel, and desirably a guide wire for guiding the conduit when it is inserted into a blood vessel, characterized in that the conduit has flexibility and self-restoring property, and the bulky portions are disposed on the conduit eccentrically, the use of the blood vessel anastomosing auxiliary tool permitting a quick and positive anastomosis between a bypassed blood vessel and a bypassing blood vessel while a blood flow remaining in a portion between the central and peripheral sides of the bypassed blood vessel is secured during a blood vessel bypassing operation, and further ensuring an unobstructed blood vessel anastomosing to allow an easier practicing operation (anastomosing) than those by conventional tools.