Friction welding was applied to cast iron known to be a poor weldability material, and the possibility of cast iron joining was examined. Friction welding was carried out using a combination of FCD370 fellow and FCD370/S45C, and measurement of the interface temperature, microstructure observation and tensile test were performed. The results showed that the highest temperature obtained at the friction interface was 1413K, partial melting occurred in both combinations, but partially fused portion was extruded by the upset pressure from interface. As ferrite that was base microstructure moved closer to the joining interface in FCD370, the base microstructure of the joint changed to pearlite. Chills, martensites, abnormality of spheroidal graphite, cracks and inclusions were not observed. The same results were obtained in the combination of FCD370/S45C either. In the tensile test, all specimens were found to break on the base materials. The tensile test after annealing showed correlation of the graphite circularity rate of the interface and the tensile strength.