Sewage sludge generated from sewage treatment plants sometimes contains chromium, and to prevent its release to the environment, it is necessary for the sludge to undergo high-temperature treatment. The molten slag treatment method is one such option. The chemical form of chromium in slag can be influenced not only by oxygen potential but also by temperature, basicity of slag, and other treatment operations. We have investigated the effect of sample basicity and cooling rate on the elution of hexavalent chromium from molten sewage sludge slag. It was confirmed that a low sample basicity (less than 0.9) and rapid cooling were effective to prevent elution of hexavalent chromium from molten slag samples. Under such conditions, elution of chromium could be maintained below the environmental quality standard (Japan) for hexavalent Cr in soil (0.05 mg Cr6+/L). XPS analysis revealed that a rigid silica network was produced at the molten slag surface for conditions of low slag basicity and rapid cooling. The formation of a rigid silica network at the molten slag surface would be effective in preventing the release of Cr6+.