A treatment instrument insertion assisting tool 1 is provided with: a connection portion 2 configured to be connected to a channel of a tool inserted into a subject an introduction port 3 for a treatment instrument inserted into the channel via the connection portion 2 a cylindrical guiding portion 4 tapered from the introduction port 3 to the connection portion 2 and a plurality of recess portions 5 provided on an outer circumferential edge of the introduction port 3, each of the recess portions 5 having such a shape that gradually increases in width from the connection portion 2 side toward the introduction port 3 side and that is recessed toward the connection portion 2 side.
