PURPOSE: To obtain an acidic aq. washing soln. for Al base-metal not having any fear of an environmental pollution and removing efficiently a stuck lubricating oil and a smut.

CONSTITUTION: The acidic washing aq. soln. for AC based-metal is incorporated with 0.5-25g/l at least one kind inorg. acid, 0.1-5g/l bromine ion and 0.1-10g/l nonionic surfactant, and can remove the lubricating oil and the Al powder, etc., sticking on the surface of Al at the time of molding work, and moreover does not contain the ion such as harmful F-, Cl-. Sulfuric acid, nitric acid and the mixed inorg. acid of sulfuric acid/nitric acid=(30/1) to (30/2) is preferable as the inorg. acid. And, HBr, KBr, NaBr, AlBr3, etc., are exeplified as a bromine ion source. Moreover, the nonionic surfactant has HLB of order of 5-15 and the mixture of that having HLB of order of 6-8 and HLB of order of 12-14 is preferable.