This paper suggests a technique that reduces aerodynamic drag of a fast cruising passenger car using actively translating rear diffuser. Actively translating rear diffuser is hidden under the trunk when a passenger car is parked and is driven slowly. And it slides out from under rear car body only at high speed driving condition not to ruin the external design of the car. 5 types of rear diffuser devices whose slid out lengths are ranging from 200mm to 450mm were installed under the car body and Computational Fluid Dynamics (CFD) analyses were performed under moving ground and rotating wheel conditions. The main purpose of this study is to explain the aerodynamic drag reduction mechanism of a passenger car cruising at a high speed via an actively translating rear diffuser device and find a length which shows the best aerodynamic performance. The diffuser device raises pressure of underbody flow with diffusing process and then underbody flow and upperbody flow mix strongly. They make strong vortices and the vortices hit the rear surface of the trunk, bringing about an aerodynamic drag reduction. Finally, aerodynamic drag of the passenger car is reduced by an average of more than 5%, which helps to improve the fuel efficiency.