Most inventory control systems show inventory levels on tabular reports as monthly figures. However, it is difficult to recognize the exact nature of inventory problems with data presented in this format. Moreover, monthly data obscures daily fluctuations, which can conceal potential problems. We developed a visualization system using “Inventory-Thumbnails” to report inventory data through small charts of daily inventory time series in a grid to enable efficient and precise problem identification. The user can assess data based on the shape of each chart and spot inventory problems at a glance. For instance, “square block” shaped charts are likely stagnant inventories, or “saw blade” shaped charts are likely fast-moving inventories. Each chart also improves clarity by showing auxiliary indicators including inventory days and safety inventory with lines or actual figures. We tested the system by asking users to find inventory problem with both the Inventory-Thumbnails and tabular reports. The Inventory-Thumbnails increased the accuracy of inventory problem detection by 40% compared with the conventional method. Additionally, one customer reduced their inventory level by 35% as a result of using our system.