Abstract: A hollow cylindrical sheath has a proximal end and a distal end. An optical viewing device is located in the sheath extending from the distal back to the proximal end. An outflow tube is slidable onto the sheath to surround the sheath and form together with the sheath an intermediate return-flow space between the outer wall of the sheath and the inner wall of the outflow tube, with the outflow tube when in position slid over the sheath tightly surrounding the distal end portion of the sheath. Clear rinsing water is introduced into the proximal end of the sheath. Turbid water is removed from the proximal end of the intermediate space. The outflow tube is provided with apertures at the distal end thereof for the flow of clear rinsing water out of the distal end of the sheath and around the end of the endoscope and then through the apertures into the intermediate space.