PURPOSE: To eliminate the possibility arising problems such as in the case of polishing and to clean the surface of a copper clad laminated sheet, by oxidizing the surface of said sheet then dipping the same in an acid soln.

CONSTITUTION: A copper clad laminated sheet is first dipped in a copper oxide treating soln. A copper acetate soln. or the like is usable for the treating soln. and while the soln. is kept heated to about 60W80°C, the sheet is dipped therein for about 3min. Then, an extremely thin copper oxide film is formed on the surface of the laminated sheet. The laminated sheet is thereafter dipped in an acid soln. Hydrochloric acid, etc. are usable as the acid soln., and the use of about 5W10% soln. of ammonium persulfate is more preferable in terms of beautiful finish. The copper oxide film formed on the surface of the copper foil is dissolved away by such dipping in the acid soln., and the rust preventive, org. material, etc. stuck on the surface of the copper foil are removed together with the copper oxide film in this stage, whereby the surface of the laminated sheet is cleaned.