PURPOSE: To achieve an effective and excellent cleaning of the drum and the like of a boiler by charging a chemical-resisting and heat insulating material through the drum except a part near the opening of the drum as a cleaning liquid flowing passage through which a heated chemical cleaning liquid is poured.

CONSTITUTION: An empty rubber or plastic bag having a chemical-resisting and heat insulating characteristics is inserted into the drum 1 and then supplied with air to expand. The drum 1 includes a baffle plate 5 provided with a number of holes 6 and arranged near the ends of pipes 2 so as to flow the cleaning liquid therethrough. Thus, the volume inside of the plant can be reduced. This results in decrease of the time required for cleaning and of the cleaning liquid, and further in an improved cleaning result.