An insertion device includes a rotor rotating around a first revolution axis, and an assist tool attached to an insertion section with covering the rotor from an outer peripheral side. An assist tool rotates together with the rotor around a second revolution axis when press force is applied from a first protrusion of the rotor to a second protrusion of the assist tool by rotating of the rotor. The first revolution axis of the rotor and the second revolution axis of the assist tool move closer to each other when the rotor and the assist tool are rotating together.