Provided is a nickel solution for forming a film that can suppress generation of hydrogen gas between a solid electrolyte membrane and a substrate while the solid electrolyte membrane and the substrate are brought into contact with each other. Specifically, provided is a nickel solution for forming a film, the nickel solution being adapted to, when disposing a solid electrolyte membrane between an anode and a substrate that functions as a cathode, bringing the solid electrolyte membrane into contact with the substrate and applying a voltage across the anode and the substrate so as to deposit nickel onto a surface of the substrate from nickel ions contained in the solid electrolyte membrane, thereby forming a nickel film containing the nickel on the surface of the substrate, supply the nickel ions to the solid electrolyte membrane. The pH of the nickel solution for forming a film is in the range of 4.2 to 6.1. The nickel solution for forming a film further contains a pH buffer solution that has a buffer function in the range of the pH and does not form insoluble salts or complexes with the nickel ions during formation of the film.