To provide storm doors having the appearance meeting the user's request, toughness, and reliability at a low price by forming a framework to which rails are fixed to an existing bay window with screws, welding or the like through attaching fittings and then installing rain shutter doors.

A plurality of attaching fittings 2 are fixed to an existing bay window with screws or the like, and rail 1 is attached and fixed with screws or welding. A corner holding fittings are attached to the rail 1 with catch clips or the like and are removed when the rain shutter door 3 is taken in or out. Size of rail is changed to match the kind and shape of the rain shutter door. A storage box for rain shutter doors not being used may be also installed. As stated above, the rain shutter doors can be attached to an existing bay window and the break of window glass may be prevented even though a disaster occurs during typhoon. Also, adhering gum tape or the like is no more needed and good appearance can be always maintained.