PURPOSE: To surface treat efficiently and stably by forming the coldest point of a low-voltage mercury-arc lamp, and regulating the temp. of the coldest point to a specified value in a device for surface treating a material to be treated placed in a treating chamber with the irradiation of the low-voltage mercury-arc lamp.

CONSTITUTION: The leading end of a low-voltage mercury-arc lamp 4 is protruded from a wall surface 1' into the outside of a treating chamber 2 and cooled by the air 5, and the temp. of the cooled coldest point is regulated to 30W50°C in a device for surface treating a material to be treated placed in the treating chamber 2 with the irradiation of the low-voltage mercury-arc lamp 4. In this way a stable treatment can be carried out even in continuous use without the fluctuations of treating effects. Since the intensity of ultraviolet rays is almost maximum at 2,537 wavelength and the intensity of ultraviolet rays at 1,849 wavelength is sufficiently high, the treating effect by ozone and remote ultraviolet light is extremely high.