PROBLEM TO BE SOLVED: To provide a method for quantitatively analyzing an Al plating liquid capable of obtaining a highly accurate analysis result and safely preparing a specimen material, while suppressing a concentration change of the Al plating liquid and the like and elongating the life of a column without giving damage to an analysis apparatus when the composition of the Al plating liquid containing AlCland alkyl imidazolium chloride is quantitatively analyzed by an ion chromatography.SOLUTION: A method for quantitatively analyzing an Al plating liquid containing AlCland alkyl imidazolium chloride includes the steps of: preparing a mixed liquid of water, an organic solvent having a SP value of 7.5 to 9.2, and an Al plating liquid; separating the mixed liquid into a water layer and an organic solvent layer; and quantitatively analyzing a cationic component contained in the water layer by ion chromatography. In preparing the mixed liquid, the water and the Al plating liquid are mixed and the organic solven
