A treatment tool insertion channel of an endoscope, which is arranged through an inside of an insertion part that includes a flexible tube and a bendable part, includes a flexible inner tube, a spiral groove formed around the flexible inner tube over an entire length of the bendable part and the flexible tube, at least one coil wound along a bottom of the spiral groove, and at least one type of filler with which the spiral groove is filled. The treatment tool insertion channel includes a first portion that includes a part inside the bendable part and a second portion at a proximal end side of the first portion, the first portion being configured more flexible than the second portion.
