Abstract: Disclosed is an airtight window, which can provide a space for enabling the window to be tilted during the fitting thereof into a window frame, and which cuts off the communication between an indoor area and an outdoor area so as to increase thermal insulation and prevent noise when installed. An upper frame of a window frame has a square C-shape with an open bottom, and has an installation end tightly contacting an installation part of a building and bent ends which extend downwardly from both ends of the installation end and which are bent inwardly so as to face each other. A movable frame of the window frame also has a square C-shape with an open bottom, and both ends thereof are formed into stopper ends so as to be disposed on and caught at the bent ends of the upper frame.