To provide a case folding method for forming a case into the flat shape to reduce the volume of the case, accelerating the reuse, recycle and the like of the case in a good manner and folding the case in an efficient and continuous manner on a carrying body by providing a simple and inexpensive constitution in a method of opening the case by using a cover and then folding the same and also provide a device thereof.

An outer cover of a case set on a folding section is opened, and then inner covers are opened and the hollow case is folded to one side to reduce the volume of the case in a case folding method. A folding device is constituted of a carrying body 4 transferring a case on which an outer cover in the side-facing posture is placed from one side to the other side, a case opening device 3 provided with an upper flap 8R and opening the outer cover of the case by a double-leaf hinged door on the folding section of the carrying body 4, a rear flap 9R and a front flap 7R for opening the front and rear inner covers N by the double-leaf hinged door and a folding device 5 for folding the case opened on the terminals of the folding section.