To protect a kelly-bar connected to an excavation bucket against a load during excavation.

Rotation of an excavation bucket 6 stops and rotation of a kelly-bar 4 also stops by load being added to the excavation bucket 6, and a casing 12 rotates and is pressed in a ground by a casing driver 11. In a cylindrical part 2, a piece 24 comes into contact with a projection 76 of a rotary member 7 by rotation of the cylindrical part 2, the rotary member 7 also stops by the stop of the kelly-bar 4, and the piece 24 is moved in a direction of separating from a center hole 23 of the cylindrical part 2 by function of an elastic member 25 by pressing force from the projection 76 of the rotary member 7. The piece 24 overcomes the projection 76 of the rotary member 7 by further rotation of the cylindrical part 2, the piece 24 moved in a direction toward the center hole 23 of the cylindrical part 2 by function of the elastic member 25, the piece 24 in the cylindrical part 2 is not engaged with the projection 76 of the rotary member 7, rotation of the rotary member 7 which the kelly-bar 4 enters stops due to idling, and the cylindrical part 2 surrounding the rotary member 7 rotates.