PURPOSE: To provide a flat diamond film by avoiding the occurrence of bowing or cracking on the film caused by tensile stress release within a diamond coating when detaching the coating from a substrate to which this coating is deposited, in the case of producing chemical vapor deposition(CVD) diamond film.

CONSTITUTION: The filming surface of the substrate the diamond coating is deposited is shaped into convex having the radius of curvature for compensating internal tensile stress forecasted in the diamond film provided by detachment from the substrate. The film detached from the substrate is substantially flattened by releasing the internal tensile stress by bowing in a direction opposed to the curve of the substrate. The radius of curvature is in the range of 50 to 250 μm per inch, molybdenum is suitable as the substrate and the thickness of the film is 1 to 2,000.