A view port of chemical vapor deposition apparatus for manufacturing semiconductor devices prevents heat loss in a chamber during a plasma deposition process. The view port includes a bracket protruding at the circumference of an opening in an electrode serving as a wall of a chamber of the apparatus, a transparent window pressed by the bracket against the wall via an O-ring, a pivoting cap for capping an opening in the bracket aligned with the window, and heat-insulative material and/or a heating element integral with the cap so as to be positioned close to the window when the cap is closed. The heating element can be a resistive heating wire or a warm air duct formed by a hose or the like. During the deposition process, the temperature of the window is maintained, thereby minimizing the tendency of polymer to adhere to the window.