A gasket having a novel structure ensuring both the gas tightness and the low-friction slidability is provided. A sliding portion of the gasket (13) includes at least a first annular projection (17A), an annular recess and a second annular projection (17B) arranged in this order in a direction rearward from a distal portion of the gasket serving as a liquid contact portion. The first projection (17A) is designed so that a first peak appears adjacent the distal portion of the gasket when the gasket is fitted in the syringe barrel (11) and a product α of a contact pressure P1f and a width D1f of the first peak is 0.4≦α≦1.0 as determined by an FEM analysis.