We investigated the effects of aroma of trees on active safety during car driving in a driving simulator. In this study, we mainly analyzed the fluctuation of driver's EEG, frequency of eye blinking and vehicle maneuver, with respect to fatigue reduction by supplying Alpha-pinene. We clarified that Alpha-pinene which is one of aroma of trees is useful to minimize the fatigue level of drivers and to keep the safety time headway to the preceding vehicle. We also analyzed the relation between the density of Alpha pinene and effectiveness in terms of reducing fatigue level. One of the results shows that supplying Alpha pinene becomes more effective as the density of that is increased, only in a condition that the annoyance level through olfactory stimulus is low. We consider that both of the annoyance level and the density of Alpha pinene operates on the effectiveness to reduce the fatigue level of driver.