Welding parameters, material flows and tool/workpiece interface phenomena have direct influences on heat generation during FSW. Although much research has investigated the important aspect of welding phenomena based on temperatures measured at specific points of workpiece, only a few experimental studies have been reported about tool temperatures because very complicated settings are required for tool temperature measurements [5] [6] [7] [8] [9]. Accurate experimental data about tool surface temperature are necessary to understand exact temperatures at the tool/workpiece interface. They are also needed to precisely validate developed simulation models by comparing measured and calculated temperatures. Therefore, a method and device for effectively measuring tool temperatures are essential to investigate FSW processes from theoretical and practical viewpoints.