A thin superconductor film is formed by mixing a gas of a complex compound between an alkaline earth metal and a partially halogen-substituted organometallic compound, a gas of an organometallic compound (halide) of a group IIIa element, and a gas of an organometallic compound (halide) of a transition metal with each other, thermally decomposing this mixture under a given oxygen partial pressure, and depositing a composite of oxides of the above-described metals on a base. In this process a novel complex compound between an organic compound having the structure (I) as the ligand and having 3 or 6 halogen atoms substituting protons and an alkali metal is preferably used as the complex compound.