A friction stir welding device (1) includes a workpiece disposition unit (2) at which workpieces (W) is disposed, a main body section (3) disposed at a surface (W4) of the workpieces (W) disposed at the workpiece disposition unit (2), a tool holding unit (4) formed in the main body section (3) to enable advance and retreat in a direction facing the workpieces (Z), which is a direction approaching and separating from the workpieces (W) disposed at the workpiece disposition unit (2), and configured to hold a tool (100), and a support medium (5) formed to satisfy a predetermined relative positional relationship in the direction facing the workpieces (Z) with respect to a first shoulder surface of the tool (100) attached to the tool holding unit (4) and configured to support the tool holding unit (4) on the surface (W4) of the workpieces (W) disposed at the workpieces disposition unit (2).