The critical change in drag occurs in the Ahmed Body at 30° of the slanted base due to the transition in the wake structure. The distinctive feature of this bi-stage phenomenon, which consists of three-dimensional and quasi-axisymmetric separation states, is that the state drastically changes. Because this feature indicates that each state is stable around a critical angle, the transition is believed to be triggered by some instantaneous disturbances. Therefore, in our previous papers, we have paid attention on the unsteady behavior of the wake to determine the trigger that induces the transition. However, the relationship between the spatial transient behavior of the wake structures and the specific frequencies has not been clarified. Then, we tried to control the degree of interaction of the trailing vortices on the downwash by changing the aspect ratio of the slanted base.In the first half, we studied the effects for typical combinations of the slant angle and aspect ratio and conducted an investigation to clarify the relationship between the geometric condition and the transition in the wake structure. In the second half, we conducted an investigation to clarify whether this instantaneous flow structure was caused by the periodic fluctuation of the downwash and trailing vortices or generated by aperiodic fluctuation, which was not involved in the periodic fluctuation. Finally, based on the above-mentioned investigations of the wake-structure behavior, we propose a hypothesis of the transitional mechanism.