The present invention relates to a syringe internal pressure cancellation type automatic drug injection apparatus, wherein, the internal pressure of a syringe, which is increased due to forward movement of a plunger at the time of drug injection, is accurately cancelled immediately after drug injection, thereby accurately injecting a predetermined amount of drug in a quantitative injection mode without the leakage of the drug due to the international pressure of the syringe and preventing the inflow of air into the syringe due to excessive backward movement of the plunger. The syringe internal pressure cancellation type automatic drug injection apparatus according to the present invention comprises a meso-gun body, a linear transfer unit for a syringe, a linear transfer unit for a plunger, a pressure sensor, and a controller. The meso-gun body includes a syringe mount, on which a syringe is mounted, provided at the upper side to be able to move in a front-and-back direction and a plunger holder, which grips a plunger for a syringe, provided at the rear portion of the syringe mount to be able to move in a front-and-back direction. The linear transfer unit for a syringe is installed in the meso-gun body, and linearly moves the syringe mount. The linear transfer unit for a plunger is installed in the meso-gun body, and moves the plunger holder forwards to inject the drug contained in the syringe into the body. The pressure sensor is installed at the plunger holder of the meso-gun body, and measures the pressure applied to the plunger holder through the plunger. The controller is installed in the meso-gun body, and controls the operations of the linear transfer unit for a syringe, the linear transfer unit for a plunger, and the pressure sensor. In addition, the controller moves the plunger backwards so that the measurement value of the pressure sensor after drug injection is equal to the measurement value of the pressure sensor before drug injection, thereby canceling t
