A 4-day superrotation 60-times faster than the planetary rotation (243 days) is observed in Venus' atmosphere. Although it has been difficult to reproduce the extraordinary phenomenon in GCMs, the superrotation is reproduced by meridional circulation and planetary-scale waves with phase velocities slower than 50 m s-1 in our improved GCM. Thermally induced waves produce equatorward momentum fluxes in the middle atmosphere, while the planetary-scale pattern of two different gravity and Rossby waves with the same frequency is found to produce the equatorward momentum flux in the lower atmosphere. These processes play crucial roles in dynamics of the simulated Venus superrotation.