A substrate 2 is autorotatably installed in a vacuum chamber 1 at an upper part thereof. MgF2 granules 3 as a film source material are put in a quartz boat 4 and mounted on a magnetron cathode 5. The magnetron cathode 5 is connected through a matching box 6 to a 13.56 MHz radio frequency power source 7. Cooling water 8 for holding the temperature of the magnetron cathode 5 constant flows on a lower face of the magnetron cathode 5. A side face of the vacuum chamber 1 is provided with gas introduction ports 9, 10 for introducing gas in the vacuum chamber 1. A shutter 11 is placed between the magnetron cathode 5 and the substrate 2. This structure provides a process enabling forming a thin film at a high speed by sputtering, especially, a high speed sputtering process enabling forming a thin fluoride film free of light absorption in a high speed by sputtering.