PURPOSE: To easily and accurately perform execution and inspection, in a method for regenerating the water supply piping in a building, by removing the rust on the inner wall of the piping and subsequently transferring the first coating material by compressed air to form a base coat film having the first color before transferring other coating material to form a top coat film having a color contrasted with that of the base coat film.

CONSTITUTION: At first, a grinding agent is sent in piping 1 by compressed air as pretreatement to remove the rust generated on the inner wall of the piping 1. Thereafter, the first coating material A is supplied to the piping 1 by compressed gas such as compressed air to form a base coat film 2 having the first color to the inner wall of the pipping 1. When the coating material A becomes a semi-dried state after coating, coating material B having a color contrasted with that of the base coat film 2 is supplied to the piping 1 to be applied to the base coat film 2 and, then the coating material B is perfectly dried, a top coat film 3 having a color contrasted with that of the base coat film 2 is formed. Since the base coat film 2 and the top coat film 3 have colors contrasted with each other, visual confirmation becomes easy, and execution and inspection can be performed easily and accurately.