On an inner peripheral surface of an assistance tool of an insertion device, a sliding surface extends from an assistance tool proximal surface toward a distal side, and when the assistance tool attached to an insertion section rotates, the sliding surface slides on a supporting surface around a longitudinal axis. A flange portion of the insertion section includes an opposed surface opposed to the assistance tool proximal surface with a gap when the assistance tool is attached to the insertion section. In a part where the assistance tool proximal surface is opposed to the opposed surface, a proximal end of an outer peripheral surface of the assistance tool does not form a step relative to the flange portion.
