A tightening head (1) is provided with at least two sockets (4i, 4ii) which are alternatively connected to an output shaft of a nut runner (3), a socket replacing mechanism (6) and a bolt supplying mechanism (7) . The socket replacing mechanism (6) is provided with a moving mechanism (61) and a posture changing mechanism (64). The moving mechanism (61) moves each socket (4i, 4ii) between a using position where the socket (4i, 4ii) agrees with an axis of the output shaft of the nut runner (3) and a waiting position which is distant from the axis. The posture changing mechanism (64) changes a posture of each socket (4i, 4ii) to an upward posture corresponding to the waiting position and to a tightening posture corresponding to the using position. The bolt supplying mechanism 7 is provided with bolt magazines (8i, 8ii) and bolt holding mechanism (9i, 9ii) . During a bolt tightening step is executed by one socket in the using position, the bolt (B) is supplied to the other socket in the waiting position.
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