The Advanced Solid Rocket (currently Epsilon Launch Vehicle) is under the development by JAXA (Japan Aerospace Exploration Agency). In this study, using our cutting-edge CFD technology, we carried out numerical analysis on the aerodynamic characteristics of the Advanced Solid Rocket at Mach 1.5. Six test cases were selected with different Reynolds numbers (wind-tunnel test and flight conditions), attack angles (0 and 5 degrees), and roll angles (0 and 60 degrees). Numerical results of the test conditions successfully reproduced the corresponding experimental data. The flight condition cases provided us with important data available at neither the experiment nor the actual flight, such as the normal force distribution, the rolling moment, Reynolds number effects, and detailed explanation of three-dimensional interactions between longitudinal vortices and the body. In particular, within the range of the wind-tunnel and flight conditions, Reynolds number turned out to have little effect on the flowfield and aerodynamic characteristics except for the rolling moment, whose difference of 19% being dominated by the leeward-positioned attitude control unit.