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