Frictions of several kinds of hydrogels, poly(vinyl alcohol) gel, gellan gel, poly(2-acrylamido-2-methylpropanesulfonic acid) gel, and its sodium salt gel, sliding on glass plates as well as on Teflon plates have been investigated both in air and in water. The frictional force and its dependencies on the load are quite different depending on the chemical structures of the gels, surface properties of the opposing substrates, and the measurement condition. The gel friction is explained in terms of interface interaction, either attractive or repulsive, between the polymer chain and the solid surface. Surface adhesion between glass particles and gels measured by atomic force microscopy showed a good correlation with the friction, which supports the polymer repulsion-adsorption model proposed by authors.