To mount and demount this dry working caisson to and from a structure in the sea regardless of a tide level, and reduce time loss at the assembly by constitut ing the dry working caisson of two vertically split split bodies and forming the dry working caisson of a breakwater functioning as a float in combination in an upper section, a work area in an intermediate section, and a ballast tank in a lower section.

When a working caisson is difficult to be installed due to a high tide level when the two vertically split split dry working caissons are moved to the lower section of a quaywall pier A, valves 9, 10 for ballast tanks 8 are opened, seawater is introduced, and buoyancy is adjusted. Since the working caissons are floated under an oblique state, repair steel-pipe piles B are held by guides 6, and the working caissons are fixed by lever blocks 11. Lower lever blocks 14 are clamped, flanges 15 are conformed, and bolts I are temporarily tightened. Air is introduced into a ballast tank 12 and the working caissons are floated, bearing blocks E are held among the working caissons and the pier A and fixed by buoyancy, and seawater is discharged by a drain pump H. Accordingly, the working caissons can be mounted and demounted to and from a structure in the sea regardless of the tide level, and time loss at the assembly can be reduced.