We developed a real-time numerical simulator for the solar wind–space–magnetosphere–ionosphere coupling system, adopting the three-dimensional (3-D) magnetohydrodynamic (MHD) simulation code developed by Tanaka. By using the real-time solar wind data, which is available from the ACE spacecraft every minute, as the upstream boundary conditions for density, temperature, flow speed, and interplanetary magnetic field, our MHD simulation system can numerically reproduce the global response of the magnetosphere and ionosphere at the same time as in the real world. We achieved real-time 3-D simulations of the solar wind–magnetosphere–ionosphere coupling system with a 44 × 56 × 60 mesh size by adapting high-performance FORTRAN language with eight CPUs on a supercomputer system located at the National Institute of Information and Communications Technology (NICT). Simulated plasma temperature and density in geostationary orbit were also plotted as an index to predict satellite charging. In addition, we present real-time virtual AE indices obtained from simulation results that directly compare with geomagnetic field activities as well as real-time plasma temperature and density in geostationary orbit. Our real-time MHD simulator now runs routinely on NICT's supercomputer system. We will present a detailed configuration of the real-time simulator system in this paper. Some examples are presented from system output to show how solar wind variations result in geomagnetic disturbances.