Disclosed is a window device for diagnosis of plasma OES (Optical Emission Spectroscopy). The window device 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 chamber and the second chamber, wherein 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, wherein the observation window opening is coaxial with the connection opening, wherein 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 chamber and the second chamber, respectively; and a transparent film moving, in a roll-to-roll manner, from the rewinder to the winder while covering an inner face of the connection opening in the housing.