A vascular anastomosis device comprises two rings with pins and an openable ring holder, which is made of a pair of components and detachably holds the rings. The end portion of a large-diameter blood vessel is inserted through a ring and the vascular wall is extended outward and impaled on the pins, the end portion of a small-diameter blood vessel is inserted through the other ring, and an incision is made from the end surface of the small-diameter blood vessel. The two rings are elliptically deformed so that the minor axis agrees with the diameter of the small-diameter blood vessel, the small-diameter blood vessel is elliptically opened and the vascular wall is extended outward and impaled on the pins, and the two rings are joined together by allowing the pins provided on the ring in one component to stick into the ring in the other component.