A gasket (3) configured in such a manner that a pusher having a male screw thread formed on the outer periphery of the tip thereof is connected to the gasket (3) through the male screw thread and that the gasket (3) slides within a syringe outer tube when the pusher is operated in the longitudinal direction thereof. The gasket (3) is provided with: a gasket body (30) having a hollow section and having formed in the inner peripheral section of the hollow section a female screw thread (33) with which the male screw thread engages； and projections (31, 32) provided in a projecting manner on the outer peripheral section (36) of the gasket body (30) so as to extend in the circumferential direction of the outer peripheral section and arranged in the axis direction of the gasket body (30) so as to be separated from each other. In a non-inserted state in which the gasket (3) is not yet inserted into the syringe outer tube, a screw thread (331) located in a portion on the inner side of and in direct proximity to the projection (32) has a smaller height than a screw thread (332) located in the remaining portion.
