A composite material substrate comprises a matrix material and a skeleton structure of a fibrous material. The matrix material exclusively participates in the thermal expansion of the substrate and the fibrous material in the mechanical strength thereof. The fibrous material constituting the skeleton structure has no influence on the thermal expansion profile of the matrix material, and the skeleton structure is loosely bonded to the matrix material at their interface. The substrate is used for solar cells. Photoelectric transfer layers of semiconductors are formed on the substrate to fabricate large-sized solar cells.