There are many unsolved molding phenomena in the hot-runner mold system In the injection molding of large or long-sized products, melts are made to flow continuously by sequentially switching gate-opening from upstream to the area contacting the flow front. Valve gates are appropriately controlled to open/close by the so called “cascade control system”, to prevent weld-lines, decrease flow pressure loss, etc. However compared to the single gate system, more complicated flow behaviors are expected to occur when the valve gate near the flow front is opened at the timing of open gates. In this study, we attempted to visualize the melt behaviors especially near opened valve gates using two sets of valve gates (VG1, VG2) for understanding what phenomena occur. First, VG1 is opened, then injected melts (transparent) from VG1 are made to approach and bypass VG2 at the timing when VG2 is opened to observe the behavior of melts (white) from VG2. This process is visualized as follows: the whole manifold channel is filled with white melts beforehand, then transparent melts are injected from the nozzle into the manifold channels at the time VG 2 is closed and VG1 is opened to replace the white melts on the side of VG1 channel with transparent ones. Through this visualization experiment, the following results were confirmed. In the case of the direct gate system, where multi-gates are located inside the mold cavity, the flow front velocity was observed to rapidly rise up to 5 or 6 times in a moment after a small amount of back flow, due to low pressure loss at the flow front area just after switching the gate open from VG1 to VG2. In the case of sub-runner systems, where multi-gates are located in the tub area outside the cavity, the later VG1 is switched to VG2, the higher the velocity peak of the melts flowing into the cavity from VG2. All these drastic velocity changes may influence upon surface transcription conditions, leading to surface defects on molded samples.