The gasket is interposed between the seal face and the seal retaining portion of the fuel cap. The gasket has a first lip for pressing against the seal face, a second lip for pressing against the seal retaining portion, and a connecting portion connecting the first lip with the second lip; and is formed with a C-shaped cross section. An assist rib projects from the inside face of the second lip. The assist rib is formed so as to press the distal edge of the first lip towards the seal face.