A medical syringe, having a syringe barrel, a piston, a plunger rod, and a plunger rod holder, wherein a barrel main body of the syringe barrel is formed to have a small inner diameter part and a large inner diameter part, the piston 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 of the barrel main body 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 by pressurizing, and the piston can be held in the large inner diameter part of the barrel main body in a state of being freely fitted thereto.
