Equatorial proton energy densities in the ring current region have been statistically investigated by compiling data acquired with the POLAR/MICS instrument (1--200 keV) in terms of the storm phases and Dst levels. The energy density is found to increase with decreasing Dst and to exhibit strong local time dependence during the storm main phase. In particular, the energy density at noon is interestingly shown to decrease during the main phase and increase during the recovery phase. A numerical simulation, which traces drift trajectories of the plasma sheet protons in the Volland-Stern type convection electric field, gives a reasonable result in comparison with the statistically obtained distribution. Those results support the scenario that the prime source of the higher energy density of the ring current protons is the plasma sheet protons whose drift motion is governed by the large-scale convection electric field most likely driven by the solar wind and IMF.