PURPOSE: To form a diamond thin film on the surface of a substrate in uniform thickness at the time of forming the diamond thin film on the substrate surface by microwave plasma CVD method by arranging a conductive member close to the substrate.

CONSTITUTION: A large-area substrate 12 to be treated is placed on a substrate holder 9 in a reaction tube, a gaseous mixture of CH4 and H2 is supplied into the tube as the raw gas, a microwave is introduced to produce plasma 16, and the C formed by the reaction of CH4 with H2 is deposited on the substrate 12 surface as a diamond thin film. Since the density of the plasma 16 is increased at the center of the substrate 12 and decreased at the periphery, the thickness of the thin film is increased at the center and decreased at the periphery, and a nonuniform thickness is caused. To prevent this phenomenon, an annular conductive member 14 is arranged above the substrate 12 at an appropriate distance from the substrate, hence a part of the plasma 16 produces local plasma 17 that concentrates on the member 14, the deposition rate of diamond at the periphery of the substrate is increased to about the same extent as at the center, and a diamond thin film 11a is formed on the entire surface of the substrate 12 in uniform thickness.