The outer tube 100 of the present invention includes two insertion holes 168 and 170 to insert an insertion part 12 of an endoscope 10 and an insertion part 52 of a treatment tool 50. Moreover, the insertion holes 168 and 170 are formed in a cross-sectional D-shape via a tabular partition wall 172 provided in the central part in a cross-section vertical to respective axial directions of the insertion holes 168 and 170. A slit 180 of a valve body 162 located in the insertion hole 170 is disposed along reference line E passing through centers C1 and C2 of respective arcs of the insertion holes 168 and 170 as seen from the axial directions of the insertion holes 168 and 170. Therefore, two leaflets 190 and 192 on both sides of a slit 180 contact with an arc edge 170B of the insertion hole 170 and a D-shaped cut part 170A of the partition wall 172 respectively when the insertion part 52 of the treatment tool 50 is internally fitted, and they are deformed into the same shape. Since there is no difference in stress caused in two leaflets 190 and 192, it is possible to secure the airtightness by the slit 180 over a long period of time.
