To significantly improve a watertight water shut-off range by omitting unevenness correction work, dispensing with a watertight bracket nor Taiburu (R) and anchor, coping with side surface active force.

In this watertight structure, a groove 10 is formed on a concrete surface A of an existing dam body, and a protrusion 20 is provided to an underwater temporary closure equipment fixing end b. A watertight rubber 30 having water swelling action is coated around the protrusion 20. The protrusion 20 of the underwater temporary closure equipment fixing end b is fitted into the groove 10 of the concrete surface A through the watertight rubber 30. Thus watertightness between the concrete surface A and watertight temporary closure equipment B is maintained, and the inside surface of the groove 10 receives water pressure load acting on the underwater closure equipment B.