To stop water easily and positively making use of water pressure applied to a water stop wall by installing a water stop seal at the bottom peripheral part of the water stop wall of truncated cone shape or truncated pyramid shape dividable and joinable at longitudinal joint faces.

Bisected water stop walls 2 are disposed around an underwater part of a bridge pier 1. Joint faces are jointed by bolts or the like, and a water stop seal 5 is installed at the bottom peripheral part of the water stop walls 2. In this case, an initial pressing water tank 6 is arranged at the upper part of the water stop walls 2, and the water stop walls 2 are fitted to the periphery of the underwater part of the bridge pier 1. Water is filled in the water tank 6, and the own weight of water in the water tank 6 is applied to the water stop walls 2 so as to make the water stop seal 5 stick closely to an underwater foundation 3 of the bridge pier 1 to stop water. The inside of the water stop walls 2 is then drained, and the water stop walls 2 are pressed further downward by water pressure difference between the inside and outside of the water stop walls 2 with slant faces so as to press the water stop seal 5 further downward to stop water positively. Repair work of the underwater part of an underwater structure such as the bridge pier 1 can thereby be performed easily by putting the inside of the water stop walls 2 in a dry state.