To provide a box body for cofferdam, which can be efficiently installed in and removed from an underwater structure such as a bearing pile and a bridge pier, and a cofferdam construction method using the same.

A box body 1 for cofferdam includes a bottomed cylinder-like box body 3 which has a bottom provided with an insertion hole 2 subjected to insertion of an underwater structure A and which is installed in such a manner as to surround an outer peripheral part of the underwater structure A, and an annular cut-off member 4 which is interposed between an inner edge of the insertion hole 2 and the underwater structure. The box body 3 comprises an openable/closable opening/closing door 6 which includes one or more door members 5 and 5 arranged in such a manner that an edge part abuts on the underwater structure A via the cut-off member 4.