To provide a cut-off tank for underwater pier reinforcing work capable of minimizing the use quantity of expensive underwater concrete and cuting-off water sufficiently.

A cut-off tank 10 underwater pier reinforcing work to be set on the foundation 2 of an underwater pier 1 is formed of a cut-off tank body 11 formed by vertically connecting, in a plurality of stages, ring units (corrugate sheet rings) 11b formed by circumferentially connecting piece bodies (corrugate sheets) 11a with a working space around the underwater pier 1; an inside ring 12 consisting of a single stage or a plurality of stages of ring units (corrugate sheet rings) situated on the lower end of the cut-off tank body 11 and provided on the inside thereof; and a connecting member 13 for connecting the cut-off tank body 11 to the inside ring 12.