PURPOSE: To prevent perfectly a pickling soln. from entering a part not to be pickled by diffusion when the inside of a boiler is pickled, by filling the part not to be pickled with glycol having a higher specific gravity than the pickling soln. before pickling.

CONSTITUTION: When the inside of a boiler is pickled with a pickling soln. 4 to remove scale and rust, a part 1 of the boiler not to be pickled is filled with glycol having a higher specific gravity than the soln. 4 before pickling. Ethylene glycol, propylene glycol, polyethylene glycol, diethylene glycol or the liks is used as the glycol 2. The soln. 4 is then passed through a part 3 to be pickled. The soln. 4 does not enter the part 1, and scale and rust are removed from only the inside of the part 3.