A molding die for efficiently molding a material for a synthetic resin molded foam at a high expansion ratio, a low density, and a high yield with the least occurrence of burrs or flashes. In this molding die, the upper die is mounted on the lower die to form an enclosed cavity therebetween. An opening portion is formed in the upper die, and a cylindrical chamber member with its head closed is air-tightly mounted on the upper die to cover the opening portion. A piston is provided in a chamber in the chamber member such that when a leading end portion of the piston is inserted in the opening portion, a micro-spacing for communicating the cavity to the chamber is formed between the leading end portion of the piston and the opening portion. A piston drive means for vertically moving the piston and for usually setting a leading end surface of the piston at a lower surface of the upper die is provided in the chamber. A pressure adjusting means is provided for evacuating air in the chamber to reduce a pressure in the chamber. In the molding die, when the air in the chamber is evacuated, air in the cavity is also evacuated through the micro-spacing.