PURPOSE: To stabilize opening and closing operation of the outermost excavation claw of a pit in the excavated pit and to surely prevent the claw mentioned above from movement in the direction of closing during the revolving excavation.

CONSTITUTION: A center cylinder body 31 is provided to the center of the front end of a cylinder rotary shaft 21. A plurality of sliding plates 33 are made to face their base ends to the inside of the cylinder body 31 and are radially provided in the direction of a diameter of the center cylinder body 31 in a slidable manner. A claw mounting body 34A is fixed to the front end of each of the sliding plates 33 and, at the same time, the outermost excavation claw 49 is mounted to the claw mounting body 34A. An operation body 32 is fitted and inserted in the direction of the axis from the front end side of the center cylinder body 31 in a slidable manner and, at the same time, the rear end of the operation body is formed in the conical surface 43, the conical surface is interlocked with the base end of each of the sliding plates 33. A taper operating surface interlocking with a conical operating surface 25 of a casing 22 is formed on an external wall surface of the rear end of the claw mounting body. When the rotary shaft 21 is pulled backward, the claw mounting body is moved in the direction of the center. When the operating body is retracted, each of the sliding plates is slid in the direction of the outside.