To improve the fitting workability and the fitting accuracy of a cover to a vehicle body, and improve the economical efficiency by simplifying a fitting mechanism.

A stepped part having a step width R to be formed inwardly in the wall thickness direction of a frame 13 and an erection piece 22 of a specified thickness to be erected in the direction parallel to the inserting direction of a resin cover from the stepped part are formed on an opening side edge of a cover fitting opening part. A side wall part 25 which is of wall thickness equal to the step width R and flush with the surface of the frame 13 when attached to the frame 13 is formed on the resin cover 18, and a rib 28 to hold the erection piece 22 in cooperation with the side wall part is integrated with the resin cover.