A gasket (4) includes a flexible packing portion (42) having a recess (43) attached to a coupler of a plunger rod, and a core member (41) harder than the packing portion (42). The recess (43) has an internal circumferential surface with an internally threaded portion (413) capable of being screwed on and thus engaging with an externally threaded portion provided on an external circumferential surface of the coupler. The core member (41) has a helical shape defining the internal circumferential surface of the recess (43) and the internally threaded portion (413) provided on the internal circumferential surface. The packing portion (42) defines a sliding surface brought into contact with an inner circumferential surface of a barrel, and the packing portion also covers an outer circumferential surface of the core member (41).
