To simplify a cofferdam work and improve the efficiency of the cofferdam work in a retaining wall repair work of an irrigation channel.

A bottom dam plate part 3 is projected downward over the whole length of a bottom of a slender hull 1 with pile installation cylinder parts 2 longitudinally provided on both broadsides respectively, movable weir plate parts 4 each having a tip edge which is a seal plate are projected respectively from a bow and a stern, and the sender hull 1 is floated on an irrigation channel 01 to be repaired. Piles 5 are driven through the pile installation cylinder parts 2 and anchored at prescribed positions, the tip seal plates of the movable weir plate parts are brought into contact with a retaining wall 02 to form a compartment within the channel and a draft is made deeper in this state by adding a weight such as water injection into the hull, and lower ends of the bottom dam plate part 3 and the movable weir plate parts 4 are made to bite into a bottom 03 of the channel up to a prescribed depth. Thereafter, water in the compartment is discharged and a cofferdam work is carried out by using the hull itself as a cut-off wall.