A liquid medicine injection apparatus capable of injecting an additional fluid after completion of injection of a liquid medicine, comprising: a cylinder elongated in a direction and having a nozzle formed at one end of the cylinder; a piston installed to be hermetically movable within the cylinder in the direction, wherein the piston defines a liquid medicine storage space to be filled with the liquid medicine introduced through the nozzle; an injector provided at the other end of the cylinder to allow the piston to be moved forward; a fluid tank for storing the additional fluid in a sealed state, wherein the fluid tank is installed within the piston so as to be movable together with the piston and is deformable by a pressure; a needle unit disposed between the nozzle and the fluid tank and installed within the piston, comprising a nozzle cap for surrounding the nozzle formed to extend into the liquid medicine storage space, and a needle secured to the nozzle cap, wherein the needle unit is configured to penetrate a portion of the fluid tank to communicate the nozzle with the fluid tank after completion of the injection of the liquid medicine with which the liquid medicine storage space is filled; and a cap member disposed between the needle unit and the fluid tank, wherein the needle unit, the cap member and the fluid tank are configured to be moved together with the piston in the cylinder, and the cap member is configured to accommodate the needle unit maintaining a predetermined gap between the needle unit and the fluid tank such that the needle does not come into contact with the fluid tank, and is configured to be deformed to expose the needle and cause the exposed needle to penetrate the fluid tank when a predetermined pressure is exerted on the cap member. a cylinder elongated in a direction and having a nozzle formed at one end of the cylinder; a piston installed to be hermetically movable within the cylinder in the direction, wherein the piston defines a liquid medicine storage space to be filled with the liquid medicine introduced through the nozzle; an injector provided at the other end of the cylinder to allow the piston to be moved forward; a fluid tank for storing the additional fluid in a sealed state, wherein the fluid tank is installed within the piston so as to be movable together with the piston and is deformable by a pressure; a needle unit disposed between the nozzle and the fluid tank and installed within the piston, comprising a nozzle cap for surrounding the nozzle formed to extend into the liquid medicine storage space, and a needle secured to the nozzle cap, wherein the needle unit is configured to penetrate a portion of the fluid tank to communicate the nozzle with the fluid tank after completion of the injection of the liquid medicine with which the liquid medicine storage space is filled; and a cap member disposed between the needle unit and the fluid tank, wherein the needle unit, the cap member and the fluid tank are configured to be moved together with the piston in the cylinder, and the cap member is configured to accommodate the needle unit maintaining a predetermined gap between the needle unit and the fluid tank such that the needle does not come into contact with the fluid tank, and is configured to be deformed to expose the needle and cause the exposed needle to penetrate the fluid tank when a predetermined pressure is exerted on the cap member.