A friction stir welding apparatus includes: a rotating shaft, on which a thread for friction stirring is formed; and a drive unit rotatably supporting the rotating shaft. A rotating shoulder configured to rotate together with the rotating shaft to generate frictional heat between the rotating shoulder and back surfaces of workpieces is fixed to a distal end of the rotating shaft. The rotating shoulder is pushed against the back surfaces of the workpieces by a pushing mechanism. A stationary shoulder penetrated by the rotating shaft and configured to hold the workpieces in a sandwiching manner together with the rotating shoulder is non-rotatably mounted to the drive unit or a member connected to the drive unit.