direction of the joining region 11 and second flat surfaces 11 c are formed on both sides only the right side is shown in fig2 a and 2b of the first flat surface 11 a while interposing stepped surfaces 11 b therebetween furthermore a first inclined surface 11 d at an inclination angle of about 30 degrees and a second inclined surface 11 e at an inclination angle of about 5 degrees smoothly continue on both sides of the second flat surfaces 11 c the rear skirt 10 basically has a symmetrical three dimensional shape and the inclined surfaces 11 d and 11 e the stepped surfaces 11 b and curved surfaces of transitional portions thereof on the joining region 11 are formed of a developable surface including the flat surfaces parallel to an axis of symmetry reference numeral 10 c in fig2 b extending in a vertical direction of the vehicle for this reason these surfaces have the same heights with respect to a pressing direction p in the direction parallel to the axis of symmetry 10 c that is the contour line with respect to the pressing direction p in any position on the joining region 11 is parallel to the axis of symmetry 10 c accordingly the protrusion 1 provided on each of the inclined surfaces 11 d 11 e on the joining region 11 is formed by an embossing process using a punch 3 with a tip having a v shaped cross section as shown in fig4 a and is thereby formed as a ridge shaped protrusion 1 a having a ridge line 1 c.