PROBLEM TO BE SOLVED: To provide an insertion instrument which can sufficiently exert a switching function of a bending length of a bending portion to reliably obtain a difference between the time of short bending in which a first bending portion is bent and the time of long bending in which both the first bending portion and a second bending portion are bent together.SOLUTION: An insertion instrument includes: a wire 40 for operating to bend a bending portion a first guide sheath 45 equipped with a first coil sheath a plurality of first flat surface parts 47a which have a first length (a) on a peripheral part of the first coil sheath in a longitudinal direction and are connected while having a separation distance of a second length (b) between adjacent end parts a second guide sheath 46 where the first guide sheath is inserted to advance and retreat, and which has a second coil sheath and a plurality of second flat surface parts 48a which have a third length (c) on an inner peripheral part of the second coil sheath in the longitudinal direction and are connected while having a separation distance of a fourth length (d) between adjacent end parts. The first length (a) is longer than the fourth length (d) (a>d), and the third length (c) is longer than the second length (b) (c>b).
