In plastic injection molding, external defects such as flow marks are serious problems. These external defects are known to occur during cavity filling processes inside the mold, and are closely related to resin flow behavior in the flow front area. In this study, the authors propose a new method for visualizing in-mold resin flow behavior using a glass-inserted mold and rotary runner exchange system capable of instantaneously switching two types of melt at the runner just before the gate. This method is capable not only of dynamic visualization, but also static visualization by the observation of cross-sections of a sample, because colored resins switched multiple times at the gate spread in layers inside the sample and freeze. Using this method and general purpose polystyrene, visualization analysis of the flow behavior of inner layer resins near the flow front in simple rectangular cavities was carried out. As a result, it was found that resins reaching the flow front are not exposed at the molded product surface immediately after the fountain-flow process, but remain along the circumference of the flow front for a long period of time, and then stretch out into a thin film-like shape to form the surface of the molded product. In addition, the inner layer resins near the flow front form a converging flow just before reaching the flow front and then form a fountain flow. It was also confirmed that this is a general phenomenon in the fountain-flow process during injection molding regardless of whether the resin is crystalline or noncrystalline. Based on these results, we built a model on the flow of inner layer resins near the flow front of general resins.