When we design a DC plasma ash melting furnace, it is necessary to evaluate some performances, such as complete ash melting, continuous slag overflow, good durability of the refractory, and high thermal efficiency. However, up to now, we have had to rely on experience to design the furnace because various phenomena are complicatedly related to each other. The purpose of this study is to establish the evaluating method to design a DC plasma ash melting furnace. In this study, we made clear the melting phenomena in a DC plasma ash melting furnace by employing simulation approaches as well as measurement of a commercially operating furnace. In measurement, we selected the infrared camera which can visualize the interior of the commercially operating DC plasma ash melting furnace through dusty atmosphere. As a result, we observed that the liquid slag surface was covered with floating ash. Also, we measured the overflowed slag temperature near the outlet of the commercially operating furnace by using an inserting-type thermocouple protected by a resistant sheath. In the numerical simulation approaches, we modeled the ash covering area visualized in the commercially operating furnace by both ash and slag as separate fluids whose properties depend on the temperature, and we carried out a three-dimensional analysis to calculate ash covering area. We carried out an electrical and thermal fluid analysis, taking account of the electrical resistance and the viscosity of slag. The validity of the simulation model was demonstrated by comparing the simulation results with the measurements of the commercial operating furnace. Both measurements and simulation results led to a conclusion that the ash covering area grew larger and the slag temperature near the outlet became higher when we increased the ash feeding rate into the furnace and that these phenomena shorten the life of the refractory. This simulation model is useful for evaluating performances when we design a DC plasma ash melting furnace.