In a friction stir welding apparatus that can control a penetrating position of a probe with respect to a processing target member to an optimum position at a time of friction stir welding by detecting a wear volume and a tip position of the probe, as a drive mechanism moves down a holder having a coefficient of thermal expansion larger than that of the probe and holding the probe, a detector can freely detect a length of a member including the holder and the probe, and a probe detection mechanism can freely detect a tip length, which is the length of the member of only the probe.