An assembly of a gasket made of a rubber-like elastic material and two opposing members and, wherein the gasket is mounted in a mounting groove provided in one of two opposing members and and is gripped between the one member and the other member so as to be in pressure contact with the two members and, the gasket comprising: a body section being inserted in the mounting groove; one support protrusion being provided on one end side of the body section in the width direction so as to be located on a bottom side of the mounting groove, and being mounted in a deep groove provided in a part of the bottom; another support protrusion; and a lip ridge protrusion being provided on the side of the body section facing the other member, and being located so that the tip thereof does not overlap the one support protrusion of the body section, wherein in a stage of assembling the gasket with the two opposing members (1, 2), the body section is displaced so as to tilt on the other support protrusion side around the one support protrusion, and a tip of the other support protrusion is in contact with the bottom and the other support protrusion is deformed thereby, and in a stage before assembly of the gasket with the two opposing members (1, 2), said other support protrusion extends in a tilted manner from the other end side, in the width direction, of the body section toward the bottom in the direction away from the one support protrusion, the tilted angle of the other support protrusion with respect to the bottom is within a range from 15 to 45 degrees and an aperture exists between the tip of the other support protrusion and the bottom of the mounting groove, wherein a surface of the other support protrusion on the side of the lip ridge portion extends in a tilted manner from the other side end in the direction away from the lip ridge portion toward the bottom. a body section being inserted in the mounting groove; one support protrusion being provided on one end side of the body section in the width direction so as to be located on a bottom side of the mounting groove, and being mounted in a deep groove provided in a part of the bottom; another support protrusion; and a lip ridge protrusion being provided on the side of the body section facing the other member, and being located so that the tip thereof does not overlap the one support protrusion of the body section, wherein in a stage of assembling the gasket with the two opposing members (1, 2), the body section is displaced so as to tilt on the other support protrusion side around the one support protrusion, and a tip of the other support protrusion is in contact with the bottom and the other support protrusion is deformed thereby, and in a stage before assembly of the gasket with the two opposing members (1, 2), said other support protrusion extends in a tilted manner from the other end side, in the width direction, of the body section toward the bottom in the direction away from the one support protrusion, the tilted angle of the other support protrusion with respect to the bottom is within a range from 15 to 45 degrees and an aperture exists between the tip of the other support protrusion and the bottom of the mounting groove, wherein a surface of the other support protrusion on the side of the lip ridge portion extends in a tilted manner from the other side end in the direction away from the lip ridge portion toward the bottom.