A part of the outer wall of the processing chamber supplied with an active gas for an intended processing forms a protruding section extending out from the outer wall into the air. An incident side window through which laser light is guided is mounted to the protruding portion. A baffle is provided inside the protruding portion for intercepting unnecessary portion of light guided in the processing chamber even if irregularly reflecting light arises, when laser light is guided into the window. An antireflection coating is coated on the air side surface of the window. A purge gas inlet port for blowing out a purge gas along the inside surface of the window is formed in the protruding portion. Therefore, a deposit caused by a process gas on the chamber inside surface of an incident side window and the surface of a stopper for beam light is prevented from producing, thereby the inside of the chamber is not contaminated and weak scattered light from a fine particle floating or falling down in the chamber can be measured.