A system of the present invention includes a storage device for distinguishably storing old objects that are objects having existed for not less than a predetermined period since being generated and new objects that are objects having existed for less than the predetermined period since being generated. Then, a new object that has existed for less than the predetermined period since being generated is selected from the storage device, and an old object having the same content as the selected new object is searched for. If such an old object is not found, the selected new object is newly stored as an old object in the storage device. If the old object is found, a reference to the selected new object is integrated with a reference to the found old object. If the old object is stored, the reference to the selected new object is changed to a reference to the stored old object.