A friction stir welding apparatus includes a displacement detecting device that detects displacement of a welding tool caused by deformation of a fitting jig, at the time of performing friction stir welding in which an arm to which the fitting jig fitted with the welding tool is fixed is moved to move the welding tool with respect to the processing target member. The fitting jig can be deformed more than the arm at the time of performing friction stir welding. The displacement detecting device has a displacement sensor fixed to the welding tool, and a reference member fixed to a fixing portion between the fitting jig and the arm to provide a reference position for detecting the displacement to the displacement sensor.