PURPOSE: To enable the formation of an oxide-based film having satisfactory superconductive characteristics by dissolving alkoxides of Mg, Ca, Sr or Ba, Sc, Y or lanthanoid, and Cu in an org. solvent together with an acid.

CONSTITUTION: Alkoxides of one or more among Mg, Ca, Sr and Ba are mixed with alkoxides of one or more among Sc, Y and lanthanoids and Cu alkoxide. The mixture is dissolved in an org. solvent such as butanol together with an acid such as acetic acid to prepare a coating agent. Each of the metal alkoxides is preferably metal ethoxide. The coating agent is stable in the soln. state and can easily form a superconductive film having a high critical temp. and a high critical current density.