A composite material comprising a bulky substrate of a Y-series 123 metal oxide crystal, and at least one layer provided on a surface of the substrate and formed of a crystal of a Y-series 123 metal oxide. The substrate may be produced by immersing a seed material in a liquid phase which comprises components constituting the metal oxide. The liquid phase contains a solid phase located at a position different from the position at which the seed material contacts the liquid phase. The solid phase provides the liquid phase with solutes which constitute the Y-series 123 metal oxide so that the solutes are transported to the position at which the seed material and the liquid phase contact, thereby permitting the Y-series 123 metal oxide to grow on the seed material as primary crystals and to obtain the bulky substrate. The layer of a Y-series 123 metal oxide may be formed on the substrate by a sputtering method, a vacuum deposition method, a laser abrasion method, a CVD method or a liquid phase epitaxy method.