To improve the mountability of a grommet in a through hole of a partition wall panel to be refitted.

The opening width dimension L of an opening section 11c cut out of the side edge 11b of the partition wall panel 11 for partitioning an engine compartment and a vehicle compartment refitted on a vehicle body frame is set at a value larger than the hole diameter of the through hole 11a continuous to the opening section 11c. The grommet 12 is formed into such a shape as to continuously shield the opening section 11c and the through hole 11a, and is fitted to the opening section 11c by passing it through the opening section 11c from the side edge 11b of the partition wall panel 11 to reduce the number of mounting processes more than conventional types, thus attaining one-touch mounting.