There is provided a process for preparing a composite material of an oxide crystal film and a substrate by forming a Y123 type oxide crystal film from a solution phase on a substrate using a liquid phase method, wherein problems such as cracking of the oxide crystal film, separation of the oxide crystal film from the substrate, and development of a reaction layer between the substrate and the solution can be minimized. The solvent for forming the solution phase uses either a BaO-CuO-BaF 2 system or a BaO-CuO-Ag-BaF 2 system, and when the substrate with a seed crystal film bonded to the surface is brought in contact with the solution to form (grow) the oxide crystal film on the substrate, the temperature of the solution is controlled to a temperature of no more than 850°C.