A medical syringe, comprising: (a) a syringe barrel having a barrel main body formed in a substantially cylindrical shape, (b) a nozzle formed at the tip of the barrel main body and (c) a flange formed at the end of the barrel main body; (d) a piston having a circular part which has an outer diameter that can be inserted into an interior space of the barrel main body; and (e) a plunger rod formed at the end of the piston in a protruding manner in the shape of a rod having a diameter smaller than the outer diameter of the circular part of the piston, and (f) a plunger rod holder having a rod insertion part into which the plunger rod can be inserted, wherein the barrel main body is formed to have a small inner diameter part on the tip side thereof, and have a large inner diameter part that has a larger inner diameter than the small inner diameter part on the end side thereof, the piston is formed such that the circular part thereof 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 of the piston can be brought into contact with an inner peripheral surface of the small inner diameter part by pressurizing, and the circular part thereof has an outer diameter equal to or smaller than the inner diameter of the larger inner diameter part of the barrel main body, the piston is integrally formed with the plunger rod by using the same or different resin material, the plunger rod holder is mounted on the end of the barrel main body so that the rod insertion part is located at a center of the barrel main body, the plunger rod is held at the center of the barrel main body in a state of being inserted into the rod insertion part of the plunger rod holder, 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; the plunger rod has a recessed groove formed to temporarily fix the plunger rod in a state of the piston being located in the large inner diameter part of the barrel main body due to that the recessed groove is engaged with an inner edge portion of the rod insertion part, and has a step part formed so as not to allow the piston inserted into the small inner diameter part of the barrel main body to move backward to the large inner diameter part of the barrel main body due to that the step part is engaged with the inner edge portion of the rod insertion part 54 and the rod insertion part of the plunger rod holder comprises a plurality of flexible boards or a plurality of flexible pins formed in a protruding manner in the inner edge portion side of the plunger rod holder in a state of being slightly inclined in the insertion direction of the plunger rod, and the plurality of flexible boards or the plurality of flexible pins are configured to permit only the movement in the insertion direction of the plunger rod and to inhibit the movement in the extraction direction of the plunger rod by being engaged with the recessed groove and the step part. (a) a syringe barrel having a barrel main body formed in a substantially cylindrical shape, (b) a nozzle formed at the tip of the barrel main body and (c) a flange formed at the end of the barrel main body; (d) a piston having a circular part which has an outer diameter that can be inserted into an interior space of the barrel main body; and (e) a plunger rod formed at the end of the piston in a protruding manner in the shape of a rod having a diameter smaller than the outer diameter of the circular part of the piston, and (f) a plunger rod holder having a rod insertion part into which the plunger rod can be inserted,