This is a method, as shown for example in FIG. 1, to form a diamond film or a diamond containing carbon film on a substrate, by causing a reactive gas introduced into a quartz reaction vessel 1 through a gas inlet 2 and outlet 5 to chemically react on the substrate 4, which is heated at 400???-1300??? C. by releasing thermoelectrons from a metallic tungsten or tantalum filament S heated by electri