To enhance the effects of sound and heat insulation by easily attaching a panel to a window panel such as a window pane in such a manner that the panel will not vibrate.

The panel comprises a panel body 1 made of an acrylic plate forming a space relative to the window panel W, a frame 2 made from vinyl chloride and closing the space, and detachable suction boards 3. When the panel is pressed against the mounting surface of the window panel W and attracted thereto by the suction boards 3, the abutting portion 2b of the frame 2 makes close contact with the window panel W and the space surrounded by the panel body 1, the frame 2 and the window panel W is closed. Thus, a double window structure consisting of the panel body 1 and the window panel W is formed to enable enhancing the sound and heat insulating effects of the window panel W.