To provide a cofferdam member assembling and sinking device which enables the assembling and sinking of cofferdam members in a easy and safe manner by using a pontoon disposed in a limited range in the case where there is limitation to a disposing position of the pontoon for assembling the cofferdam members.

The assembling and sinking device is provided with plural winding devices 2 installed on brackets 12 protruded on beam parts 11 of a bridge pier, a circular elevation frame 3 horizontally suspended from the winding devices, and circular rails 6 disposed and supported on the elevation frame 3 to be rotatable in a horizontal plain through plural plain trolleys 4. It is also composed of a rotation drive mechanism 5 disposed on the elevation frame 3 to drive the circular rails 6 to rotate in the horizontal plain to the elevation frame 3, and an installation ring 7 of cofferdam members 8 disposed on the circular rails 6.