Abstract: A ventilation device installed for windows, which is located at the inner side of a window and has multiple inner ventilating openings on its side. The upper frame of the ventilation device is mounted on the window's upper frame, and Frame 1 of the device includes the lower frame, which is mounted on the window glass. The ventilation device also includes a flap, which can make a swing motion pivoting on a hinge structure, and Frame 2 with multiple outer ventilating openings on its side, which is attached to the outer bottom of the upper frame and the outer top of the lower frame in a detachable manner. It also includes an elevation part making the flap's opening and closing motions, a supporting part securing spaces for those movements, and an opening switch part originating the movement of the elevating parts. Frame 1 contains a hinge receiving part that can hold the swingable flap, and the opening switch located at the lower portion of the window.