Authors tried to butt-weld a mild steel plate to a magnesium alloy plate by the solid state welding using a rotating pin. This study investigated the effects of a pin rotating speed, the position for the pin axis to be plunged (pin offset) on the strength and the microstructure of the joint. The main results obtained are as follows. Butt-welding of a steel plate to a magnesium alloy plate was easily and successfully achieved. The maximum tensile strength of a joint reached about 70% of the magnesium base metal tensile strength and the fracture path was along the joint interface. When a pin rotating speed was slow, some defects appeared in the magnesium matrix due to the insufficient plasticization of the magnesium. The joint strength increased with the pin rotating speed. This seems to be because the plasticization of the magnesium was increased and the pressure for pushing the plasticized magnesium onto an activated faying surface of the steel increased. However, the excessive increase of the pin rotating speed caused the ignition of the magnesium, resulting in the decrease of a joint strength. At the pin offset of 0.1mm toward steel, steel fragments scattered in the magnesium matrix in the form of a small piece that had no influence to the joint strength. By contrast, larger offsets over 0.2mm produced the steel fragment continuous and parallel to the weld interface in the magnesium matrix, and the joint strength decreased.