The present invention provides a gasket excellent in properties such as sliding properties and resistance to liquid leakage. The present invention relates to a gasket whose sliding surface includes annular projections, the gasket having polymer chains immobilized on at least part of its surface, the annular projections including a first projection that is nearest to a top face, and a bottom side projection that is farthest from the top face, the first projection and/or the bottom side projection having an annular flat portion with a width of 0.1 mm or more.
