The manufacturing sites of injection molded products are often faced with defects which disfigure the appearance of molded products such as silver streaks, weld-lines, and flow-marks. In the case of molded products with mirror surface, one such external defect is the tree-ring-like whitening defects appearing on the surface. From experience, we know that this defect occurs conspicuously with PBT/PET when the mold temperature is high. Although this molding defect is thought to occur in the demolding process, the mechanism by which it occurs is not clear. For this reason, in this study, we investigated the conditions by which tree-ring-like whitening defects occur and reviewed the generation mechanism by visualizing the mold separation process. We found that when the mold is forcibly separated from the molded product while they are still tightly adhered together , the surface of the molded product that is tightly adhered to the mold wall deforms, causing demolding to progress intermittently, resulting in the whitening defects. In addition, characteristic tree-like patterns are thought to be formed due to this separation of the surface of the molded product from the mold wall while it warps from the external circumference.