A window device for diagnosis of plasma OES (Optical Emission Spectroscopy), is disclosed and includes a housing including a first chamber and a second chamber horizontally adjacent to each other; a connection opening defined in one face of the housing and between the first and second chambers, the connection opening faces toward an opening through which light of plasma from a plasma chamber is exposed to the connection opening; an observation window opening defined in an opposite face of the housing opposite to the connection opening, the observation window opening is coaxial with the connection opening, the light of the plasma transmits through the observation window opening, and is incident to a light receiver of an OES sensor; an observation window positioned inside the observation window opening and in the housing; a winder and a rewinder installed in the first and second chambers, respectively; and a transparent film.