PURPOSE: To form a homogeneous and thick diamond film even on the major flank of a member just as on the cutting face by placing the member to be coated with a thin film on a support having a carrying surface smaller than the bottom face.

CONSTITUTION: The support is continuously or stepwise expanded from the carrying surface toward the bottom face. An insulating material transmissible to a microwave is preferably used as the material for the support. A protrusion is preferably extended from the carrying surface, the inner surface of the protrusion is abutted on the non-angular part of a member to be coated with a thin film, and the member is placed on the non-angular part. A diamond thin film is then formed on the member by the vapor phase method. The area ratio of the support to the bottom face is preferably adjusted to about (1:2)-(1:20).