Choosing appropriate sites for wind turbines is extremely important in Japan because the spatial distribution of wind speed is highly complex over steep complex terrain, which is abundant in Japan. The author's research group has been developing an unsteady CFD software package called RIAM-COMPACT【○!R】. This package is based on an LES turbulence model. In this paper, to examine the accuracy of RIAM-COMPACT【○!R】, numerical simulations of uniform non-stratified airflow past a three-dimensional cube were performed. The analysis primarily focused on airflow characteristics in the wake region. The results from the simulation with RIAM-COMPACT【○!R】 were compared to those from a commercially-available CFD software package (STAR-CCM+) and were found to be in good agreement.