a pair of gaskets is integrally formed with seal lips corresponding to each other and extending in the longitudinal direction of the gasket , and side portions disposed on either sides or one side of the seal lip and having a height lower than that of the seal lip . the plate - shaped attachment member includes a through - hole provided at a position where the side portions of the pair of gaskets communicate with each other so as to be formed therethrough in the thickness direction , and the pair of gaskets is integrally formed through the through - hole . an opening shape of the through - hole is set to be an elongated shape in the longitudinal direction of the gasket so as to make a width dimension of each of the side portions as small as possible .