PURPOSE: To make it possible to carry out a stabilized cleaning operation by installing a board for making gas flow on the surface in a laminar flow and controlling the thickness of a gas layer flowing between the board and the surface.

CONSTITUTION: A quartz board 3 through which ultraviolet rays penetrate is so laid out as to face a wafer so as to form a gas flow layer 15. A plurality of nozzles are welded to the quartz board 3 to supply ozone. Furthermore, there is laid out a nozzle 5 which lets the gas flow in order to measure the thickness of the layer whereas a pressure gauge 6 is installed on the way of a pipeline to the nozzle 5. The pressure of the pressure gauge 6 varies with the size of an outlet cap of the nozzle 5. A pressure range, which is equivalent to a control gap range, is predetermined based on the relation previously established between the cap and the pressure gauge 6. The predetermined pressure range is input into a control circuit. A pulse motor-driven mechanism for vertically moving a stage 2 operates in such a fashion that the gas flow layer 15 is fixed by an output signal of the value of the pressure gauge 6.