In order to investigate the durability of an Ir-based alloy tool, the FSW of 304 stainless steel plates was performed under various welding conditions, and the mechanical properties of the joints were measured. Defect-free joints are possible under certain conditions and the mechanical properties of the joints are the same as the base material. When the rotation pitch is about 0.3, the degree of tool wear of the Ir-based alloy tool with a 1.4-mm probe height, which was used for the FSW of a 304 stainless steel plate with a 2-mm thickness was less than or equal to that of the PCBN tool with a 2-mm probe height, which was used for the FSW of 304 stainless steel plate with a 9-mm thickness. The result of the tool life test in this study showed that the Ir-based alloy tool enabled the FSW of 304 stainless steel over a 75-m length.