A closure device closes an end opening (38) of an instrument channel (50) of an endoscope. A coupling sleeve (86) is for connection with the end opening, and has a guide lumen (70) for passage of an instrument. Two sealing walls (65) are disposed to project from the coupling sleeve in a shiftable manner in a proximal direction in relation to the entry of the instrument, for contacting one another to seal the guide lumen in an openable manner, to prevent fluid from passing around the instrument. A housing sleeve (60, 66) contains the sealing walls. A coupling flange (84) is disposed to project from the sealing walls in the proximal direction, for coupling the sealing walls to the housing sleeve. A regulating projection (90) is disposed to project from an inner surface of the housing sleeve, for regulating the coupling sleeve in the proximal direction.
