Construction materials are stocked as structures in some years, but overage and unnecessary structures can cause new material flow to be wasted. In the near future, a huge overage stock that was built during a period of rapid growth will cause the new flow to become waste. Therefore, it's important to see "where" and "how much" material flow may emerge in any given city. An Over-flow Potential Map, or OPM, offers a way to visualize Material Flow related to the construction sector. In the course of estimating regional Material Flow using GIS, many map layers must be calculated. An OPM is produced by taking the 'recycled In-flow layer' away from the 'Outflow layer'. The Over-flow potential is concerned with the age of structures, construction materials used, and maintenance needed. OPM can show both the density of over-flow material, and the contents of over-flow in cities.