A plunger for a syringe that includes a barrel includes: a gasket mounting portion located at a distal end of the plunger; a disk positioned at a proximal end of the gasket mounting portion; a shaft extending proximally from the disk, wherein the shaft comprises a plurality of slats extending proximally from the disk; a pressing portion located at a lower end of the shaft; and a plunger inclination-prevention unit that is located at a distal end of the shaft and is configured to inhibit the plunger from inclining inside the barrel of the syringe. The plunger inclination-prevention unit comprises one or more axially inclined plates, each having a first end disposed on the disk, extending obliquely proximal with respect to a central axis of the shaft, and having a second end disposed on one of the slats.