A device for injecting a fixed quantity of liquid medicine, the device comprising: an upper housing through which a first cavity is vertically formed; a lower housing configured to be connectable to a lower portion of the upper housing, with a second cavity being vertically formed through the lower housing; a cap disposed in an upper portion of the first cavity, and configured such that at least a portion thereof linearly reciprocates in the first cavity; a first pushing block configured to receive a force from a lower end of the cap and thereby move downwards, with a hollow portion being vertically formed in the first pushing block; a second pushing block provided in the first cavity under the first pushing block to be spaced apart therefrom by a predetermined distance; and a pushing bar extending to a predetermined length to be inserted into the upper housing and the hollow portion of the first pushing block and supported at a first side thereof to move downwards by a predetermined distance as the first pushing block moves downwards; wherein a lower portion of the pushing bar is configured such that at least a portion thereof is inserted into a syringe, wherein the pushing bar comprises a linear ratchet gear which is longitudinally formed on a side surface of the pushing bar, the first pushing block comprises a first pawl engaging with the ratchet gear, and the linear ratchet gear and the first pawl are configured to move the pushing bar downwards and limit an upward movement thereof, wherein the second pushing block comprises a hollow portion formed to allow the pushing bar to be vertically inserted therein, and a second pawl engaging with the linear ratchet gear to move the pushing bar downwards, and wherein the pushing bar is moved downwards by a preset distance for each vertical reciprocating movement of the cap to push a piston of the syringe; characterized in that the device further comprises a first spring provided between the cap and the first pushing block; and a second spring provided between the first pushing block and the second pushing block. an upper housing through which a first cavity is vertically formed; a lower housing configured to be connectable to a lower portion of the upper housing, with a second cavity being vertically formed through the lower housing; a cap disposed in an upper portion of the first cavity, and configured such that at least a portion thereof linearly reciprocates in the first cavity; a first pushing block configured to receive a force from a lower end of the cap and thereby move downwards, with a hollow portion being vertically formed in the first pushing block; a second pushing block provided in the first cavity under the first pushing block to be spaced apart therefrom by a predetermined distance; and a pushing bar extending to a predetermined length to be inserted into the upper housing and the hollow portion of the first pushing block and supported at a first side thereof to move downwards by a predetermined distance as the first pushing block moves downwards; wherein a lower portion of the pushing bar is configured such that at least a portion thereof is inserted into a syringe, wherein the pushing bar comprises a linear ratchet gear which is longitudinally formed on a side surface of the pushing bar, the first pushing block comprises a first pawl engaging with the ratchet gear, and the linear ratchet gear and the first pawl are configured to move the pushing bar downwards and limit an upward movement thereof, wherein the second pushing block comprises a hollow portion formed to allow the pushing bar to be vertically inserted therein, and a second pawl engaging with the linear ratchet gear to move the pushing bar downwards, and wherein the pushing bar is moved downwards by a preset distance for each vertical reciprocating movement of the cap to push a piston of the syringe; characterized in that the device further comprises a first spring provided between the cap and the first pushing block; and a second spring provided between the first pushing block and the second pushing block.