Using a newly developed mixed state evaluation mold, the disintegration and distribution of fiber bundles in long-fiber reinforced resin pellets inside melts injected from the molding machine nozzle were investigated by visualizing the density of the luminance change from the backlight source. This mixed state evaluation mold consists of two parallel glass blocks for observation on both sides of the melt flowing channel. The molten fiber-reinforced resin flowing through the gap between the glass blocks can be videoed by a high-speed camera and the captured video images analyzed in time series. When long glass fiber reinforced resin pellets are melted and the fiber bundles inside are disintegrated by the rotational screw in the heating cylinder, distribution is thought to be poor due to incomplete fiber-bundle disintegration and fiber fibrillation. Using this evaluation mold and long glass fiber bundles, the change in the state of distribution of long glass fiber bundles with time was quantitatively analyzed, focusing on luminance changes and the state of fiber pilling which corresponds to the disintegration of fiber bundles in long-sized pellets. Distribution was analyzed by calculating the area ratio of disintegrated fiber bundles over the total area. In the case of low fiber content, the disintegrated long fibers could be clearly identified in the injection process. However, in the case of high fiber content, the complexity of the optical path of light makes the analysis difficult. Depending on various conditions such as fiber content ratio (up to 50wt%), injection speed, screw rotation speed, melt temperature, and pressure, fiber-bundle distribution could be clearly analyzed. The results clarify that the state of disintegration of fiber bundles during the screw plastication process can be quantitatively and time-sequentially analyzed using this evaluation mold, and confirm the usefulness of the analytical method.