In die casting, it is necessary to decrease porosity defects by implementing some kind of countermeasure. Porosity defects in die castings are roughly divided into two types : shrinkage porosity and gas porosity. Different measures are used for different porosity defects. Porosity can be visualized using an X-ray CT scanner, but their differences cannot be quantitatively discriminated. On the other hand, the blister test can detect gas porosity. The blister test makes gas porosity, but not shrinkage porosity, expand significantly. However, the positions and postures of die castings in X-ray photography cannot be fitted exactly before and after the blister test. In addition, the blister test causes casting products to deform due to thermal stress. Therefore, it is difficult to compare the volumes of the porosity. We propose a method of discriminating porosity defects by comparing volumes before and after the blister test. Accurate discrimination is achieved by calculating the volume ratio between the identified porosity before and after the blister test. Eventually, we demonstrated the effectiveness of the system for discriminating between gas and shrinkage porosity defects by applying it to actual die casting.