An insertion device includes an insertion section extended along a longitudinal axis, and an assist tool detachably attached to the insertion section. The assist tool is actuated with being attached to the insertion section, thereby causing a propulsion force for movement along the longitudinal axis to act on the insertion section. A moving member movable between a first position and a second position is provided in one of the insertion section and the assist tool. The moving member is held at the second position due to a pressure from the other of the insertion section and the assist tool, in a state that the assist tool is located at the specific position relative to the insertion section.
