To provide a temporary cofferdam work caisson and its installation method by which water pressure control and a sufficient water stopping property can be assured.

In the upper open type temporary cofferdam work caisson 1 formed into a ring-like shape in a plan view by joining a pair of assembling units 2 in a state of facing each other, a fitting recess 10 is formed at the bottom 9 of each of the assembling units 2. The front surface of the fitting recess 10 is an inclined surface 11 whose upper part is inclined inward. A water stopping packing 3 movable up and down along the inclined surface 11 is arranged on the inclined surface 11.