An insertion device includes a shape-variable tube elastically returning while the shape-variable tube bends at a tube bending radius of a tube radius boundary value or more, and a shaft rotating around a shaft axis inside the shape-variable tube so that the shaft transmits a driving force to drive a motion section from a first extending direction toward a second extending direction. The shaft elastically returns while the shaft bends at a shaft bending radius of a shaft radius boundary value or more, and rotates without being deformed while the elastic return is impossible when the shape-variable tube bends in an elastically returnable range.