A longitudinally-endless gasket mounted into a groove portion formed in one of members to provide sealing between this one member and another of the members when they are fastened to each other. The gasket has a sectional shape corresponding to a sectional shape of the groove portion with a ratio (H/W) between a height (H) and a width (W) of the sectional shape being 0.8 to 5.0. The gasket is provided with a plurality of large projecting portions at intervals of 30 to 100 mm and a plurality of small projecting portions at intervals of 5 to 15 mm in a longitudinal direction of the gasket. A width (R2) of each large projecting portion is larger than a width (X) of the groove portion by 0.01 to 0.9 mm. A width (R1) of each small projecting portion is smaller than the width (X) of the groove portion by 0.01 to 0.6 mm. A filling rate of the groove portion by the gasket when the members are fastened to each other is 80 to 100%.