Connecting device for electrical junction boxes

A connecting device for electrical junction boxes, including a sleeve member that can be inserted in a first hole of a first wall of a first box and in a second hole provided in a second wall of a second box to be joined to the first box. An annular gasket is fitted around the sleeve and is located between the two walls. The sleeve has a first end provided with a plate that is internal to the first box and a second end provided with an abutment means that engages the inner side of the second wall. The insertion of a wedge member, which is applied between the plate and the first wall, gradually moves the plate away from the inner side, pushing the walls against each other and compressing the gasket between them.