A prefilled syringe assembly adapted to exert a high piston-slip-off-preventive force and less liable to damage the piston. A prefilled syringe assembly comprising a syringe, and an attaching flange threadedly joined to the syringe, the attaching flange being intended to prevent slip-off of the piston. The attaching flange and plunger are desirably formed with a reuse-preventive projection and a recess adapted to engage each other when the plunger is inserted to the medicinal liquid discharge position within the syringe. The inner surface of the syringe on the open end side may be formed with an air bleed groove which defines a clearance between it and the piston when the piston is positioned on the open end side of the syringe.
