A model describing the frictional force produced when a polymer gel is sliding on a solid surface has been proposed from the viewpoint of solvated polymer repulsion and adsorption theory at a solid surface. General relations for the frictional force f expressed as functions of the normal loading P, sliding velocity v, the polymer volume fraction ?, or the elastic modulus E of the gel, etc., have been derived by applying scaling relations to the model. For the repulsive case, f is ascribed to the viscous flow of solvent at the interface and f is theoretically demonstrated to be proportional to the sliding velocity v and the normal pressure P when the pressure is smaller than the elastic modulus of the gel. For the attractive case, in addition to the hydrodynamic friction, the force to detach the adsorbing chain from the substrate appears as friction. When v is not very large, f ? v. At an intermediate velocity, f has a velocity dependence less than linear, depending on the strength of adsorption. At a higher sliding velocity, f ? v again since the hydrodynamic friction becomes predominant at this stage. The theoretical results coincide well with some experimental observations that confirm the essential feature of the model.