The goal of this study is to develop an actively translating rear diffuser device to reduce the aerodynamic drag experienced by passenger cars. The feature of this device is hidden under the rear bumper ordinarily not to ruin the external design of the car and slips out backward under the high-speed driving condition. By this study, a movable arc-shaped semi-diffuser device is designed to maintain the streamlined automobile rear underbody configuration. It's installed under the rear bumper of a passenger car.Seven types of rear diffuser devices whose positions, slid out lengths and widths are differing with the basic shape installed in the rear bumper section of a passenger car and performed Computational Fluid Dynamics (CFD) analyses under rotating wheel and moving ground conditions. The main purpose of this study is that explains the aerodynamic drag reduction mechanism of a passenger car via an actively translating rear diffuser device at a high speed driving condition. The base pressure of the rear surface of passenger car is increased by install the rear diffuser device, because it prevents the low pressure air coming through the underbody from directly soaring up to the rear surface of the trunk. At the same time, it increases a diffusing effect to underbody flow which lowers the velocity and raises the pressure, bringing about an aerodynamic drag reduction.Finally, the automobile aerodynamic drag is reduced about an average of more than 4% in 450 mm length with the base configuration. And also maximum 7% of drag reduction accomplished changing the shape of the rear diffuser device into various types. At the end, the diffuser device was applied to the YF SONATA developed and produced by the Hyundai Motors Company. The drag reduction was achieved up to 5.2% in the speed range of 80 km/h ~ 120 km/h. In addition, owing to generation of negative lift by the diffuser device, it is expected the driving stability improved in high speed condition.