To provide a device for effectively joining caissons with one another for an underwater tunnel which always swings.

A ball joint 27 is interposed between joint end parts 20a of caissons to be joined to each other through the intermediary of a water- unpermeable elastic material which is expandable in a direction in which the caissons are extended, and a water stopping material 34 such as a rubber gasket is interposed between recess and projection joint surfaces 30a, 31a. Further, a primary water stopping material 25 which is axially expandable is stretched between the caissons outside of the ball joint 27.