To easily change the position of a spud for stabilizing a working pontoon into a position not impeding passage.

A spud 11 freely elevatably supported and stabilizing a working pontoon 1 by stabbing the bottom of a body of water with its end is freely movably supported by a cylindrical spud holder 12. A derricking device 13 changing the position of the spud holder 12 from an upright position to a lying position is provided, and if there is the fear that the spud 11 may collide with a bridge girder or the like and impede the passage of the working pontoon 1, the derricking gear 13 is operated to lay the spud holder 12 down.