Let us open the first pages of the books on polymer or rubber physics. 1–3 These books usually begin with the explanation of the origin of polymer chains’ elasticity. They do not originate from energetic elasticity mainly possessed by solid-state materials such as a metallic spring but from so-called entropic elasticity. The entropic elasticity is the consequence of an essential property of polymer chains, i.e., a single polymer chain being composed of a large number of unit constituents. Thus, even a single polymer chain can be treated by statistical mechanics. The calculation with a boundary condition that these constituents are bonded to each other to form a chain structure gives the final formula of entropic elasticity. This is the story, developed during the era when a great advocate of polymer science, professor H. Staudinger, proposed that a single polymer takes the form of a linear string or chain molecule.