PURPOSE: To efficiently dissolve scale contg. oxide or hydroxide of transition elements without corroding the base material by cleaning the base material with an acid or chelate agent-contg. cleaning liquid added with metallic powder of Zn, Mn, Al, etc.

CONSTITUTION: The metallic powder of at least one kind among Zn, Mn and Al is added to the cleaning liquid contg. an acid such as hydrochloric acid or chelate agent such as EDTA at the time of dissolving the scale which sticks to the base material of a heat exchanger, etc., and contains the oxide and/or hydroxide of the transition elements such as Fe, Mn and Co with the above- mentioned cleaning liquid. The cleaning liquid is preferably prepd. to 0W12pH and may be used for cleaning by coating the same in the form of slurry on the base material. The reduction dissolving capacity of the cleaning liquid is remarkably improved by the addition of the above-mentioned metallic powder and the scale is efficiently dissolved away at a low temp. of, for example, an ordinary temp. W about 40°C. The corrosion of the base material is thus decreased.