A medical syringe, having a syringe barrel (2), a piston (3), a plunger rod (4), and a plunger rod holder (5), wherein a barrel main body (21) of the syringe barrel is formed to have a small inner diameter part (24) and a large inner diameter part (25), the piston (5) is formed such that the circular part is comprised of an elastic material and has an outer diameter larger than the inner diameter of the small inner diameter part (24) of the barrel main body (21) so that the outer periphery of the circular part can be brought into contact with the inner peripheral surface of the small inner diameter part (24) by pressurizing, and the piston (3) can be held in the large inner diameter part (25) of the barrel main body (21) in a state of being freely fitted thereto.
