To simply and easily prevent a fall of double sliding doors from a lower rail due to a wind pressure and vibration by a method wherein an outside lock piece is mounted between the upper rails of the double sliding doors and floating of the double sliding doors are suppressed.

Rails 2 and 2 formed in an upper side part 10a of a frame body are slidably fitted in respective rail grooves formed in the upper ends of double sliding doors 12 and 13. An outside lock piece 15 having a proper length is pressed in a space between the rails 2 and 2 and fixed at an upper side 10a. A gap between the upper ends of the double sliding doors 12 and 13 and the outside lock piece 15 has size shorter than a depth of fit-in between the lower ends of the double sliding doors 12 and 13. This constitution prevents a fall of the double sliding doors through suppression of floating of the double sliding doors 12 and 13 due to a wind pressure and vibration. Further, the outside lock piece 15 may be fixed at an upper side part 10a by using a pressure- sensitive adhesive double coated tape, an adhesive, and a screw. Further, the outside lock piece 15 is mounted on the upper side part 10a without removing the double sliding doors 12 and 13 and the length and the mounting position of the outside lock piece 15 may be properly set.