A backflow preventing structure of a blood sampler capable of preventing backflow of blood positively without causing a significant increase in the number of components and cost. The backflow preventing structure of a blood sampler comprises a hub(3) as a tubular body having an internal channel(3a) extending from a first end part(3b) toward the second end part(3c) side, and a bottomed tubular backflow preventing member(6) which is composed of a material having rubber elasticity and contained in the internal channel(3a), has an opening opened toward the first end part(3b), and is closed on the second end part(3c) side. Outer circumferential wall of the backflow preventing member(6) is secured liquidtightly to the inner wall of the internal channel(3a), and a cut(6c) is made in the backflow preventing member(6) at a part closer to the second end part side than the secured part.
