PURPOSE: To obtain a high-quality diamond film with the internal fault reduced in good yield by previously dividing the surface of a synthesis substrate into plural synthesis surfaces by a demarcation line such as groove and synthesizing a diamond film on each surface in the vapor phase.

CONSTITUTION: A diamond film is synthesized in the vapor phase on a synthesis substrate 1. In this case, the surface of the substrate 1 is previously divided into plural synthesis surfaces by a demarcation line consisting of a slow- synthesis means such as a groove 2. By this method, the internal stress in the diamond growth stage and the stress generated by the difference in thermal expansion coefficient between the diamond and substrate are remarkably reduced as compared with the case when a diamond film is synthesized on the surface of an undivided large surface of a synthesis substrate. Accordingly, a high- quality diamond film with the irregular cracks minimized and the internal fault reduced is produced in higher yield.