PURPOSE: To reduce the air resistance of a vehicle, suppress the temperature rise in the vicinity of an exhaust pipe and a muffler, and reduce the dimension and weight of the whole device, as for an undercover device for a vehicle.

CONSTITUTION: At the lower part of an automobile body 10, an undercover 12 which covers the undersurface side of the automobile body 10 and reduces the air resistance is installed. At the intermediate part 12A in the vehicle longitudinal direction of the undercover 12, a plurality of slits 14 as heat radiation holes are formed, keeping a prescribed interval, at the center part in the car width direction. These slits 14 are formed under the tunnel part of a center floor pan through which an exhaust pipe extending from an engine passes. On the upper side of the slit 14, a slide cover 28 on which the silts having nearly the same shape to the slit 14 are formed, keeping the equal intervals, is installed so as to be slidable in the vehicle longitudinal direction, and the effective opened port area of the slit 14 is varied by relatively shifting the slide cover 28 in the vehicle longitudinal direction, for the slit 14.