PURPOSE: To easily perform the visual confirmation of coating irregularity, in a method wherein coating material is transferred by compressed air to form a film to the inner wall of a piping system, by forming a base coat film having one color by one coating material and subsequently forming a top coat film having a color contrasted with that of the base coat film by other coating material.

CONSTITUTION: At first, a grinding agent is sent in piping 1 by compressed air as pretreatment to remove the rust generated on the inner wall of the piping 1. Subsequently, one coating material A is supplied to the piping by compressed gas such as compressed air to form a base coat film 2 having one color to the inner wall of the piping 1. After coating, when the coating material A becomes a semi-dried state, 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 dried to form a top coat film 3 having a color contrasted with that of the base coat film 2. Since the base coat film 2 and the top coat film 3 have colors contrasted with each other, visual confirmation becomes easy and a pinhole or coating irregularity can be simply discovered.