To provide a backflow prevention tool capable of reliably preventing backflow of a liquid and mixing of bubbles, a method for manufacturing the same and a medical tool such as a syringe or an infusion container or the like that can reliably prevent backflow of a liquid, or mixing of bubbles.

The backflow prevention tool 10a is made by internally inserting one open end 17 of a second cylindrical member 16 having a channel 19 penetrating in an axial direction (x) into a first cylindrical member 11 having a body 12, a valve 13 and a latching section 14 and fastening both. The body 12 and the latching section 14 in the first cylindrical member 11 and the valve 13 are integrally formed by injection molding. The open end 17 butts the valve 13, and blocks flow of a liquid in the first and second cylindrical members 11 and 16, and easily compressively deforms, when liquid pressure is applied from the second cylindrical member 16 side and forms a channel of a liquid perpendicular to the direction of compression deformation in a space with the open end 17.