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A numerical simulation technique was implemented to automatically optimize the plunger velocity to reduce the defects occurring during die-casting by considering the number of free surfaces and potential energy of the molten metal. In pressure die-casting, the most common defect is porosity that is formed by air entrapment in the following two stages - the first is the injection of the molten metal into shot sleeve, and the second is the filling of the molten metal into the cavity. The latter phenomenon has been investigated in detail by various numerical simulation codes, but there is limited information concerning the former. In order to the simulate flow pattern in the shot sleeve, a moving boundary method is incorporated into the conventional filling simulation system. In addition, the plunger speed and the acceleration time for the various pre-filled levels of molten metal in the sleeve are determined by automatic optimization.