For a substrate (W) placed in an airtight processing vessel (1), plasma is generated by introducing a microwave to a radial line slot antenna (4). Conditions are set such that the pressure in the processing vessel is in the range of from 7.32 Pa to 8.65 Pa, the microwave power is in the range of from 2000W to 2300W, the distance (L1) between the surface of the substrate and an opposed face of a raw-material supply member (3) is in the range of from 70 mm to 105 mm, and the distance (L2) between the surface of the substrate and an opposed face of a discharge gas supply member (2) is in the range of from 100 mm to 140 mm. Under these conditions, a raw-material gas consisting of a cyclic C5F8 gas is activated based on energy of the microwave. Consequently, film-forming species containing C4F6 ions and/or C4F6 radicals in a greater content can be obtained. Thus, a fluorine-added carbon film excellent in the leak properties and heat stability can be securely formed.