Abstract: In order to create a treatment device for clearing up fistulas with which fistulas can be treated as sparingly as possible, and where the functions of the adjacent anatomical structures are to remain as intact as possible, the invention proposes a fistula blocker for clearing up a fistula passage with a plug-like closure device, at least somewhat insertable into a fistula passage, which has a bearing surface which extends at least in part circumferentially transverse to the direction of insertion and which can be brought into contact with the wall of a fistula passage, and where the closure device is provided with flexible application string which can be inserted into the fistula passage, the latter being designed as a drainage pipe.