To prevent a relay connector from being possibly pulled out, together with a connecting plug when pulling it out, in a connection structure in which the connecting plug is connected via the relay connector.

When connecting an inserting and pulling type connecting plug 1 to a relay connector (U link 2), detachably mountable to a prescribed spot, an energizing mechanism enabling disconnection of the connecting plug from the relay connector, while pressing the relay connector in its mounting direction, when pulling out the connecting plug is provided in the connecting plug 1. The energizing mechanism may be structured by a cover member 5 externally mounted to the connecting plug and made movable in the inserting and pulling direction of the connecting plug; may be structured by an elastic member interposed between the connecting plug and the relay connector and projected toward the side of the connecting end surface of the connecting plug; and may be structured by a bent member comprising a working piece, interposed between the connecting plug and the relay connector, and an operating piece, bent from this work piece at a prescribed angle and extended at an acute angle to the side face of the connecting plug.