Abstract: A structure of a flap part for covering a rail part when opening a sliding door of a vehicle, includes a door flap part including an installation hole, a door flap guide, door trim flap links, elastic members, and a flap, wherein the sliding door is supported by a roller arm, and the rail part, which is externally exposed when the sliding door is opened, is covered by the flap part, and wherein at a time of opening the door, a roller provided on the roller arm comes into contact with a door flap guide and pushes the door flap guide to ensure a movement space for the roller arm.