In order to improve the fuel consumption in the real world, it is important to reduce the increase in aerodynamic drag with on-road turbulence due to atmospheric wind and other vehicles. It is well known that the aerodynamic drag is significantly affected when passing another large vehicle. However, there are few studies about aerodynamics of a vehicle passing or passed by another vehicle focusing on the drag increase, though many studies have been reported about transient aerodynamics of a vehicle passing another vehicle with regard to the running stability. In this study, we investigated physical mechanisms of the drag increase caused by the passing vehicle, in order to establish a reduction technique of such drag increase. Firstly, we conducted on-road measurements of two sedan-type vehicles passed by a truck. Their aerodynamic drag estimated from the base pressure measurement showed different increment when they were passed by the truck. This result raises the possibility to reduce the drag increase by a modification of local geometry. Then, we conducted wind tunnel measurements of simplified one-fifth scale vehicle models in quasi-steady state, in order to understand flow mechanisms of the drag increase systematically. Two main factors of the drag increase, which are pressure distribution and change of wind direction generated by the passing vehicle, were identified from the measurement results. Finally, we conducted the wind tunnel measurements of a scale model modified to reduce the latter factor. As the result of the measurement, we verified the reduction technique of the drag increase.