High-strength, low-frictional-coefficient gels are produced by adding a third component to a double-network gel (DN gel), of either a weakly crosslinked network or non-crosslinked linear chains (to produce, respectively, a triple-network gel (TN gel) and a DN-L gel, see Figure). All the gels are highly transparent. The DN-L gel shows a fracture strength as high as 9 MPa and its frictional coefficient is as low as 10(-5) under an extremely high pressure of the order of sub-MPa.