Abstract: Metal materials made of iron (Fe) or the like are butted together at a joint, and a probe of a rotary tool is inserted into the joint. The rotary tool is rotated while being moved in the lengthwise direction of the joint, whereby the metallographic structure of the joint is stirred, and the metal materials are joined. While the rotary tool is being rotated and moved, liquid CO2 is supplied to the joint and the rotary tool. Electromagnetic valves are suitably opened and closed so that nozzles discharge the liquid CO2 from the rear in the movement direction of the rotary tool. Supplying the liquid CO2 to the joint and the rotary tool reduces wear of the rotary tool, and also increases the joining strength of the joint.