A friction stir welding apparatus includes a rotating tool provided with an upper rotating body arranged to be axially adjustable in position by an upper actuator, a lower rotating body integral with a rotating main rod extending through the upper rotating body and arranged to be axially adjustable in position by a lower actuator, and a stirring part formed in the rod. The upper rotating body and the lower rotating body clamp therebetween a joint of members to be joined placed with their end faces butting each other. The stirring part is rotated to stir the joint by frictional heat to join them. The rotating tool includes a slide plate attached, through a thrust bearing, to one or both of end portions of the upper rotating body and the lower rotating body facing each other to provide good finished surfaces of the joint.