The nano-friction of ⁴He films adsorbed on graphite substrate has been reviewed. From experiments of the quartz-crystal microbalance (QCM), the following properties of the sliding motion of films and the friction have been revealed: (1) At two-atom and three-atom thick films, the frictional force of the boundary of the first and second atomic layers is much smaller than that of the boundary between the film and the substrate. (2) At two-atom and three-atom thick films, the frictional force remains metastable at low temperatures after switching of the driving force, before relaxing to the value. (3) At four-atom thick films, the solid atomic layer underneath the superfluid layer stops the sliding motion on the oscillating substrate. From these observations, it is concluded that a non-uniform sliding of the second atomic layer occurs with respect to the the first atomic layer, and the frictional force of films is related to the resistance force for the motion of this local density change.