In the JR maglev system, trains are operated by the electrodynamic force between ground coils and on-board superconducting magnets (SCMs). When a train is running, SCMs are vibrated by the electromagnetic disturbance that occurs when the train passes over the ground coils. Such a phenomenon generates frictional heat in the SC-coil. It has been surmised that this frictional heat is caused by the relative microscopic slips between the structural components of the SC-coil. The dual aims of this study were firstly to elucidate the mechanism of frictional heat inside an SC-coil, and secondly to reduce frictional heat. A fretting test was performed at liquid-helium-cooled boundaries between the structural components of an SC-coil. The test results demonstrated that frictional heat had occurred at the boundary between an epoxy-impregnated superconducting coil and the coil fasteners. Furthermore, a method to reduce the frictional heat inside an SC-coil was considered by using a correlation between the hardness of the frictional surface and the amount of frictional heat. Then, an effect to reduce the frictional heat was demonstrated by means of a liquid-helium-cooled fretting test.