A connecting device for coupling a rinsing system to a channel of a surgical instrument to be cleaned, including: a connecting element； a supply-side coupling apparatus having a holding channel to receive the connecting element extending within the holding channel in a longitudinal axial direction in a connected state, and which is provided at the end of a connecting channel that communicates with the channel, wherein the holding channel is connected to the rinsing system for feeding a rinsing liquid to the channel, wherein the coupling apparatus comprises a sealing element； and means for hydraulically and/or pneumatically actuating the sealing element； wherein the sealing element is elastic； and the sealing element reduces a cross-section of the holding channel in a region of the sealing element by means of an expansion transverse to the longitudinal axial direction) to provide a liquid-tight connection of the connecting element to the coupling apparatus.
