PURPOSE: To simplify work safely and to reduce a construction period and a cost without requiring any barge for conveying waling steel materials and any crane ship used for unloading the steel materials to a specific position from the barge.

CONSTITUTION: Walings 1a-1n are formed of an air-tight box material, and portions of the number of steps mounting the walings 1a-1n are piled up in a step-like state and are towed on the sea. Earth retaining steel sheet piles 3 used for a cofferdam method in the sea and the walings 1a-1n are separated from upper steps and are connected to specific positions in accordance with the lowering of the water level in the cofferdam of the walings 1a-1n piled up in a step-like state.