This paper presents fundamental properties of the storm-time ring current on the basis of the numerical simulations. This simulation model solves spatial and temporal evolution of the ion distribution in the magnetosphere by tracing the bounce-averaged drift trajectories. The tracing is performed under the dipole magnetic field and the time-dependent Volland-Stern-type convection field. After tracing particles, we calculate the differential flux, the plasma pressure, and the current density. The magnetic disturbance induced by the ring current is directly calculated from the Biot-Savart integral over the whole three-dimensional distribution of the calculated current density. We examined following subjects during the magnetic storms; the causes of the ring current buildup, the electric current distribution, the causes of the ring current decay, the energy composition of the plasma pressure, the response time of the plasma sheet density to the solar wind density, and the diamagnetic effect. This simulation suggests the following results: (1) The major variation of corrected Dst is mainly due to the changes in both the convection electric field and the plasma sheet density. (2) The Dessler-Parker-Sckopke relation overestimates the corrected Dst by a factor of 2.5--4. (3) The storm-time ring current buildup is insensitive to the plasma sheet temperature for the temperature above 3 keV. (4) The ions with energies around 15--40 keV at L~4--6 in the dusk region mostly contribute to the perpendicular pressure. (5) The equatorial magnetic fields are dramatically distorted by the diamagnetic effect. The grad-B drift trajectories under the distorted equatorial magnetic field can be classified into four patterns.