An insertion instrument includes an insertion section extending from a proximal side to a distal side along a longitudinal axis, and the insertion section includes an assistance tool attaching portion to which an assistance tool is attached. A dimension index indicating a dimension from a distal end of the insertion section is provided on an outer surface of the insertion section. A position index provided at a reference position, which is spaced from a proximal end of the assistance tool attached to the assistance tool attaching portion at a predetermined dimension to the proximal side, on the outer surface of the insertion section, the position index being an index indicating a position is the reference position.
