A gasket is configured to be mounted in a gasket mounting groove that opens to the radially inside of a mounting member provided with a hydraulic passage, and is provided with a back-side hydraulic passage serving as a gap on its groove bottom surface. The gasket includes a gasket body with a reinforcement ring buried therein and a flange provided to extend from one axial end of the gasket body toward the radially outside. The gasket also includes a one-end-side seal lip that is provided to extend from the one axial end of the gasket body toward one side in an axial direction and the radially inside, and is configured to come in contact with one axial side surface of the gasket mounting groove when the gasket body is mounted. The gasket also includes an other-end-side seal lip that is provided to extend from the other axial end of the gasket body toward the other side in the axial direction and the radially outside, and is configured to come in contact with the other axial side surface and a bottom surface of the gasket mounting groove when the gasket body is mounted.