In the No.3 hot rolling mill, Chiba Works of JFE Steel, the endless rolling process, in which roughly rolled bars are joined and rolled continuously in the finishing mill, has been put into operation for mild steels. On the other hand, this process is not applied to high-tensile-strength steels, because alloy elements contained in the high-tensile-strength steels, such as silicon and manganese, form oxide layers at the joining interface, disrupting the joining process. In laboratory joining tests, by heating until the joining surface melted, the oxides layer was discharged from the interface and steels that form oxide with a low melting temperature were successfully joined. FEM flow analysis revealed that the viscosity of oxide is the most important parameter for discharging. The low melting temperature of oxide leads to the low viscosity, resulting in the discharge of the oxide layer.