PURPOSE: To roughen the surface of an Al sheet as a base material for printing to an excellent grain form by immersing the Al sheet into an electrolyte having a specific compsn. and subjecting the Al sheet to an AC anodic oxidation.

CONSTITUTION: The Al or Al alloy sheet to be used for the base material for printing is immersed in the electrolyte having the following compsn. and is subjected to electrolytic surface roughening by supplying AC current in order to roughen its surface to the grain form. An aq. acid mixture soln. which is composed of 2.5 to 20g/l HCl and 1 to 100g/l in concn. of the acids selected from one kind among oxygen acids, such as glycolic acid, lactic acid, α- oxybutyric acid, mandelic acid, glyceric acid, malic acid and tartaric acid, dicarboxylic acids, such as oxalic acid, malic acid, succinic acid, glutaric acid and adipic acid, and succinic anhydride and maleic anhydride as their derivatives is used as the electrolyte. The surface of the Al sheet is anodically oxidized and dressed to the grain from by supplying AC thereto for 30 seconds to 5 minutes at an AC voltage of 5 to 42V and a current density of 1.5 to 10A/dm2 as electrolytic conditions.