The authors developed a rotary runner exchange system which is able to instantaneously switch between flow resins of two colors before the gate and proposed a new visualization method which enables observation of the internal flow behavior. In this study, the authors confirmed that resins of the initial filling stage are stretched into a horn shape along both sides of the cavity. This side-edge flow area found to distribute along both sides of the rectangular cavity, and the distribution state varied with the change in the aspect ratio between the cavity width and thickness. With square cross sections, the side-edge flow area was confirmed to divide into four corners. The same flow behavior does not occur with circular cross sections. With rectangular cross sections, a solidified layer and a nonflow layer grow consistently from the cavity wall during the flow process. This causes the shape of the residual flow area around the center of the cross section to change during the flow process. As a result of flow history of regions, where shear flow selectively continues over a long time, a horn-shaped side flow is thought to be formed near both sides of the cavity or its four corners. POLYM. ENG. SCI., 51:721-729, 2011. (C) 2011 Society of Plastics Engineers