PURPOSE: To eliminate the reduction of actual production time and prevent a cleaned mold from getting dirt by a method wherein an inner cylinder is descended to fix a lower mold to a lower pressure plate during using the mold while the inner cylinder is ascended to lift up a mold upon replacing the mold.

CONSTITUTION: When a mold replacing device is released and a mold fixing and lifting device 5 is operated to pull down an inner cylinder 4, locked to a lower mold 7, upon operating the mold, the bottom force 7 is centered correctly at the center of the machine by a mold centering mechanism 6 while descending. The mold fixing and lifting device 5 outputs a force in a direction of descending the bottom force 7 and fixes it regidly to the lower pressure plate 1. In case the mold is removed, fixing of an upper pressure plate and top force 10 is released and the bottom force 7 is put on the top force 10. The mold fixing and lifting device 5 is operated upwardly whereby the fixing of the bottom force 7 and the lower pressure plate 1 is released and the forces 7, 10, which are held by the inner cylinder 4, are ascended. According to this method, a time necessary for replacing the mold may be shortened and an operator will never enter into the inside of the machine.