To provide a coffering structure capable of conducting sufficient cutoff even when sheet piles are arrayed in a row or even when the sheet piles are deflected by water pressure or the like, while being capable of construction in a short period at a low cost, and its constructing method.

The coffering structure is constituted, in such a manner that mounting plates 6, 6 are opposed mutually and fixed to a pair of two sheet piles 5, 5 in a plurality of the sheet piles 5 respectively, while a cutoff member composed of a flexible material such as a rubber is stretched between these mounting plates 6, 6. In the constructing method, a plurality of the sheet piles are built to a ground in a row, combining these mutual joint sections, grouting is executed to separately combined joint sections, and the mounting plates are made opposed mutually and fixed to a pair of the two sheet piles respectively, while the cutoff member is stretched between these mounting plates.