PURPOSE: To uniformize the light attenuation factor in a window by flowing gas, which absorbs the light, between two window materials through which the light is transmitted and changing the flow rate of this gas.

CONSTITUTION: Fused quartz or synthetic quartz is used as window materials of optical windows 1A and 1B through which the light is transmitted, and gas which absorbs the light is flowed between optical windows 1A and 1B and the flow rate of this gas is controlled. A gas supply system consists of a supply system of gas which absorbs the light and that of (carrier) gas which does not absorb the light, and these two kinds of gas of both systems are joined and are connected to a gas introduction ports 3 and 3R and are let to flow in opposite directions. Since boundary layers are formed in opposite directions, the sum of thickness of respective boundary layers is uniform throughout a wide range, and the light attenuation factor in windows is uniformized.