PURPOSE: To enable the formation of a minute, hard and uniform thin film, by putting-ON an ultraviolet ray source lamp by applying a high frequency power source, and making the lamp radiate a continuous light whose luminous intensity does not become zero.

CONSTITUTION: From a high frequency power source 7, high frequency power is applied to an ultraviolet ray source lamp 1, both ends of which are provided with electrodes 3 for applying high frequency power, through a lead wire 4 and a coaxial cable 5. At this time, in order to prevent the discharge from being unstable because of a load resistance component and the like of the lamp 1, a matching apparatus 6 is put in the next stage of the high frequency power source 7 to get matching. When a frequency of the high frequency power source is set at 13.56 MHz, the fluctuation of luminous intensity of the ultraviolet ray at the time of radiation of the ultraviolet ray lamp is hardly observed, and almost continuous radiation can be obtained. Thereby, at the time of the photochemical reaction for thin film formation, the ultraviolet ray is continuously irradiated, so that the quality of a formed film becomes uniform and compact.