To provide an underwater temporary cofferdam installation that can omit unevenness adjustment work.

An underwater temporary cofferdam installation is provided vided with a caisson installed on a reservoir side concrete face 2a of a dam body, a movable cover fitted to the upper face of the caisson, and watertight hardware 16 provided at the leg part of the caisson. The watertight hardware 16 is provided with synthetic rubber 17 in a relative position to the concrete face 2a so that load transmission mortar 10 filled in a relative part to the concrete face 2a does not come in direct contact with the concrete face 2a. A watertight hardware fixing member 12 fitted to a watertight hardware 16 is protrusively provided with a position adjusting bolt, and a liner 15 is interposed between the concrete face 2a and the watertight hardware fixing member 12 adjusted in space by the position adjusting bolt. Unevenness adjustment work can therefore be omitted. A follow-up margin to unevenness of the concrete face can be taken as large as possible, and there is no need to chip off the load transmitting mortar after removing the underwater cofferdam installation so as to be able to reduce installation cost.