A die opening and closing device for a movable die plate and a fixed die plate includes a toggle link disposed between the movable die plate and a die tightening housing, a cross head for flexing the toggle link, and a motor-driven ball screw penetrating through the die tightening housing and inserted into the cross head, and this die opening and closing device is provided with die thickness adjusting means for adjusting the die thickness S when the die is tightened by extension of the toggle link, and ball screw rotation control means for controlling constant the cracking gap, and a foamed resin molding machine is presented, together with its operating method.