A new plastic joining method for fixing bars with a hot-forged plate was proposed, in which steel bars maintained at room temperature are directly indented into a high-temperature steel plate, as reported in a previous paper. To examine the validity of the developed plastic joining method, the optimum conditions of the plastic joining method are investigated using an SCM435 steel bar and an A5052 aluminium plate on a mechanical or AC servo-controlled press. It is possible to indent the bar to the plate without the buckling or plastic deformation of the bar at room temperature, and the attained shear bonding stress of the bar–plate is approximately 70% of the shear strength of the plate material. The bonding mechanism of the proposed plastic joining method is discussed from the viewpoints of plate seizure and mechanical clamping associated with the process.