PROBLEM TO BE SOLVED: To have both of airtightness and low slidability.SOLUTION: A prefilled syringe includes: a gasket 4 disposed so as to be slidable back and forth in an inner hole 3H of a barrel 3； a plunger rod 6 including a screw shaft 6B projecting to a front end, the screw shaft to being threadedly engaged with a female screw 5A of a screw hole 5 of the gasket 4； and a diameter expansion member 7 capable of expanding the gasket 4 by the threaded-engagement with the female screw 5A and enhancing a contact pressure between the barrel 3 and the gasket 4. The diameter expansion member 7 is removable from the female screw 5A, and the expansion of the gasket 4 is released by the removal.SELECTED DRAWING: Figure 2
