A connecting device for media lines including a coupling element, an insert part and a fastening element. The coupling element has a connection opening and a fluid channel; the insert part can be inserted into the connection opening; and the insert part can be fastened to the coupling element by the fastening element. The fastening element also has at least one control surface. When the insert part is inserted into the connection opening under the action of an installation force, the insert part acts upon the control surface in such a way that the fastening element is moved against a restoring force. If the installation force is sufficient, the insert part slides past the control surface and the fastening element interlockingly locks the insertion part. If the installation force is not sufficient, the fastening element presses the insert part back out of the connection opening.