A laminating water-based surface treatment agent according to the present invention includes a chitosan derivative, a carboxyl compound having at least one carboxyl group in a molecule, and water. The content mass ratio of the chitosan derivative/the carboxyl compound falls within a range of 1.0/0.5 to 1.0/3.0. The laminating water-based surface treatment agent of the present invention can form an undercoating excellent in HF-resistance, electrolyte resistance, and solvent resistance, and also can sufficiently secure laminating adherence for a long term under more extreme environments required for a packaging material for a battery case or a tab lead for, e.g., a lithium ion secondary battery for automobiles.