PURPOSE: To improve the heat insulating ability without hindering the function of a window by inserting the heat insulating material having small cells into the inside of a double window to keep the heat insulating ability, and providing a driving device for winding it at the time of natural lighting and the ventilation.

CONSTITUTION: A heat insulating window 1 consists of a fitting frame 2, a double window 3, a housing part 4 for housing the heat insulating material 10 and a driving device, etc. The double window 3 is formed with two pieces of glass 3A and a space 3B, and the heat insulating material 10 made of multiple air foam 11 are located. A winding device 5 is provided in the housing part 4 for housing the heat insulating material 10, and is connected to a motor 6. Next, the heat insulating material 10 is freely drawn out from and put in the double window 3 with the driving device 7, and while the air is supplied/ discharged by a pump mechanism. Natural lighting and the ventilation can be thereby performed as same as ordinary window by housing the heat insulating material 10 in the housing part 4.