PURPOSE: To prevent or reduce a clogging phenomenon in a pipe by forcing under pressure a liquid to which gelatin has been added into between two pistons connected by a string or the like in the pipe to form a nonelastic plug body.

CONSTITUTION: An appropriate liquid is heated and gelatin is added to the liquid, which liquid is forced under pressure into between two pistons 4, 4 which are connected together by a wire 5 or a string and inserted into a pipe 1. In this case, a proper hole is provided at an upper part of the pipe 1 so that the hole is located between the pistons 4, 4, whereby a liquid is forced into the pipe through the hole. Thereafter, the liquid in the pipe 1 is cooled to form a nonelastic plug body in a jelly-like state 6. As a result, even in the case of a long-distance piping, downwardly sloped piping, a piping having a large number of bents, high pressure water washing work can be conducted easily in a reliable manner even by an unskilled worker.