Aluminum castings are widely used in transportation equipment because of their high strength to weight ratio, heat conductivity, and corrosion resistance. In the past, heat-treated steel and cast iron were used for engine parts that required wear resistance, but the demand for weight reduction has gradually increased, and nowadays most of the large parts that make up an engine are made with Aluminum. If the large aluminum casting parts requires wear resistance, the steel parts may be partially included. The method is to wrap it at the time of casting or to accurately press it in. However, in order to achieve functionality, it may be better to partially bond the steel. Therefore, for the purpose of bonding a wear-resistant steel to the cast aluminum AC4C alloy, the influence of the bonding conditions and the shape of the joint was investigated in this study. This time, using the resistance welding method, we focused on the reactions and microstructural changes that occur near the interface between steel and aluminum. In order to obtain a large bonding strength, it is important to remove the oxide film on the aluminum surface, control the influence of the intermetallic compound layer generated at the interface between steel and aluminum, and increase the amount of plastic flow on the joint surface. But, in bonding by resistance welding, each process of energization, heating, deformation, and joining is performed in a very short time, and it is difficult to understand the effects and interrelationships between each factor. It has also become clear that as the target area to be bonded becomes larger, it becomes more difficult to bond under the same conditions at all interfaces of steel and aluminum. For this reason, we decided to start with a simple shape and then join test pieces that are gradually complicated and have a large area shape. In addition, considering that a partial difference in adhesive strength will occur, an undercut groove has been added as a countermeasure.