The HPDC (High Pressure Die Casting) manufacturing method has a high productivity because the molten metal is injected at high speed into the mold. On the other hand, it has been considered that, when molten metal is injected at high speed, the amount of gas entrained into the part increases, causing deterioration of the quality. In recent years, researches have been investigating the relationship between J value, which is an index showing the state of atomization of molten metal, and the quality of a molded product. J value is defined by geometric parameters of the injection port (gate) and the injection gate speed. In the investigations on tensile strengths of mass-produced products, we found that there were products with high strength of about 1.5 times as high as usual. In addition, products with high tensile strength were molded under the conditions with a higher J value, and gas distribution conditions in the products were also different. In order to investigate the mechanism of the enhancement of the strength, firstly, we tried to observe the atomized state of the molten aluminum during injection. Since it is difficult to directly observe the state of the molten metal in the cavity, the in-cavity space ahead of the injector opening was replaced with a open space, and the state of gushed molten metal was photographed. High speed aluminum molten metal could not be photographed in detail by reflection photography under halogen light, so transmission photography was applied under high-speed blinking of a pulse laser. Through this observation, it was found that atomization of the aluminum molten metal occurs due to collisions of the high speed molten metal against the preceding low speed molten metal. From these experiments, we identified the possibility that this generation of minute particles enhances the properties of HPDC products.