An endoscope system includes an endoscope and a cleaning adapter or connector. The endoscope has a port device or end sleeve, to which the cleaning adapter is coupled. The cleaning adapter includes a connector housing and a coupling sleeve, which has a male tapered portion. The connector housing includes a female thread (fastening device) and a guide surface. The port device includes a flange and a male thread. The guide surface guides the port device by contacting the flange. Thus, a male tapered surface or tapered sealing surface is axially aligned with a female tapered surface or tapered bore surface. The male tapered surface tightly contacts the female tapered surface by threaded engagement between the male and female threads. Accordingly, air-tightness or liquid-tightness can be ensured even in use of a plurality of connectors couplable selectively for connecting a related apparatus.