The flexible tube has an outer layer portion having a tubular shape covering a spiral tube of natural length and having an outer layer portion having a large inner diameter with respect to the outer diameter of the helical tube, and the outer diameter of the helical tube and the outer diameter of the outer layer portion Such that the spiral tube remains in a non-contacting position with the inner circumferential surface of the outer layer portion contacting on the outer circumferential surface in the radial direction of the spiral tube during the period from the linear state to the maximum curved state due to the gap due to the difference with the inner diameter The diameter of the helical tube and the diameter of the outer layer portion are set.可撓管は、自然長の螺旋管を覆う筒状の形状を成し螺旋管の外径に対して大きい内径を有する外層部を有する外層部を具備し、螺旋管の外径と外層部の内径との差による隙間により、螺旋管が直線状態から最大湾曲状態までの間、螺旋管の径方向の外周面上で接触する外層部の内周面とは非接触となる箇所が残るように螺旋管の径と外層部の径が設定される。
