To enhance the rigidity of an outer frame and an inner frame so that they will be more resistive to distortion, and only small damage may be caused to a person in case the inner frame is opened or dropped to hit the person.

The outer frame and the inner frame are each formed by bending a shape into a quadrilateral and connecting both ends together, and four corners of each of the outer frame and the inner frame are roundly curved. Thus compared with conventional outer and inner frames which are formed by connecting four shapes together at respective corners, the rigidity of the present outer and inner frames is enhanced, and by virtue of the roundly curved four corners, they cause only small damage even in case of hitting a person.