PURPOSE: To reduce air resistance and suppress the temperature rise in the vicinity of a muffler, as to the undercover rear part structure of a vehicle.

CONSTITUTION: At the rear part of a rear undercover 24 arranged on the undersurface of an automobile body 10, an inclined surface 24B which is gently inclined over a vehicle toward the vehicle rear end part is formed. On the inclined surface 24B, an opened port part 42 is formed, and the vicinity of the rear end part 36A of a muffler 36 projects outside the automobile body 10 from the opened port part 42. Accordingly, the applied air resistance is reduced by the vicinity of the rear end part 36A of the muffler 36 which projects from the opened port part 42, and the max. temperature in the vicinity of the muffler 36 can be reduced.