An endoscope apparatus for endoscopic treatment is disclosed. The apparatus comprises an endoscopic insertion tube comprising an outer surface including an outer rail formed longitudinally thereon and extending along a portion of the insertion tube. The outer rail has a predetermined shape. The insertion tube comprises an inner channel formed therethrough. An endoscope apparatus for endoscopic treatment is disclosed. The apparatus comprises an endoscopic insertion tube without a working channel and an outer surface including at least one outer rail formed longitudinally thereon and extending along a portion of the insertion tube. The outer rail has a first predetermined shape. At least one medical device includes an outer wall having at least one mounting unit formed thereon along a portion of the length of the outer wall. The mounting unit has a second predetermined shape cooperable with the first predetermined shape. The second predetermined shape is formed to complement and slidably cooperate with the first predetermined shape of the outer rail for slidably attaching the medical device thereto.