To form a film such as a diamond film on a substrate without causing warpage of the substrate by holding the substrate with a substrate holder with the both surfaces of the substrate exposed, disposing a film forming device on each side of the substrate, and simultaneously forming films of a specified material on both surfaces.

A substrate 10 is fixed by a substrate holder 3 in a vacuum chamber 1 with its surfaces 10a, 10b exposed. The chamber parts 1a, 1b are formed on the respective sides of the substrate 10. Each chamber part 1a, 1b is a film forming device equipped with a vacuum pump 2a, 2b, gas inlet 6a, 6b, microwave power supply 7a, 7b, waveguide 8a, 8b, resonator plate 9a, 9b or the like and has a symmetric structure with respect to the substrate holder 3. The substrate 10 is controlled to a specified temp. by using pipes 4, 5 to introduce and discharge cooling water and a high frequency coil 11 to heat the substrate, and Ar, H2, CH4 or the like are introduced through the gas inlets 6a, 6b to produce plasma by microwaves so as to simultaneously form diamond films on both surfaces 10a, 10b of the substrate 10 under same conditions.