High thermal conductivity PPS is expected to be applied to cooling cases of in-vehicle parts because of its light weight and high heat dissipation. However, this resin has a peculiar flow that can causes breakage at the flow front during filling, resulting in short shots and cracks on the surface layer at the end of the flow. In order to suppress these defects, it is necessary to elucidate the filling behavior and obtain the requirements for optimization. We thus here intend to visualize the filling behavior of high thermal conductivity PPS. To achieve this goal, we develop an in-process visualization system to reveal both thermal and kinetic behavior of the resin during filling in the cavity. In the system, a sapphire prism glass is carefully inserted in the mold. Sapphire glass is adopted since it shows high strength, high heat conduction, and high infrared transmission. Both a high-speed visible camera for kinetic behavior and infrared camera for thermal behavior are used. With the developed system, we successfully obtain the filling behavior of thermal conductivity PPS for the first time. Visualization experiments show that the temperature of conventional PPS gradually decreases from the tip of the flow to the rear. On the other hand, the temperature of the high thermal conductivity PPS drops sharply from the tip of the flow to the rear, and the breakage at the flow front near the cavity wall is generated. Our interpretation is as follows: Since the ductility of the high thermal conductivity PPS largely decreases because of the rapid temperature drop, the flow front near the cavity wall can be easily broken when it is stretched during filling. To suppress the formation of this breakage, we modify the cavity shape and verify the suppression effect.