A drug injection device comprising: a plunger which is connected to an ampoule filled with an injectable solution and applies pressure to the injectable solution in the ampoule; and a manual plunger handle which is connected to one side of the plunger and applies pressure to the plunger by means of manual labour of an operator; characterized by further comprising: an automatic plunger pressurizing unit which is selectively connected to the plunger in a position adjacent to the plunger and automatically applies pressure to the plunger by means of electrical power; wherein the automatic plunger pressurizing unit comprises: a plunger driving portion which provides the electrical power for applying the pressure to the plunger; a connection gear portion which is selectively connected to or released from the plunger by a gear method; and a gear operating portion which operates the connection gear portion, wherein the connection gear portion comprises: a worm gear which is attached to the plunger driving portion; and a rack gear which is formed on an outer surface of the plunger and gear-engages with the worm gear, wherein the automatic plunger pressurizing unit further comprises a unit casing which integrally supports the plunger driving portion and the worm gear, wherein the gear operating portion comprises: a pusher which operates the unit casing so that the worm gear gear-engages with the rack gear when sliding pressure is applied in one direction; and an elastic member which is connected to the unit casing and elastically biased in a direction in which the worm gear is released from the rack gear. a plunger which is connected to an ampoule filled with an injectable solution and applies pressure to the injectable solution in the ampoule; and a manual plunger handle which is connected to one side of the plunger and applies pressure to the plunger by means of manual labour of an operator; characterized by further comprising: an automatic plunger pressurizing unit which is selectively connected to the plunger in a position adjacent to the plunger and automatically applies pressure to the plunger by means of electrical power; wherein the automatic plunger pressurizing unit comprises: a plunger driving portion which provides the electrical power for applying the pressure to the plunger; a connection gear portion which is selectively connected to or released from the plunger by a gear method; and a gear operating portion which operates the connection gear portion, wherein the connection gear portion comprises: a worm gear which is attached to the plunger driving portion; and a rack gear which is formed on an outer surface of the plunger and gear-engages with the worm gear, wherein the automatic plunger pressurizing unit further comprises a unit casing which integrally supports the plunger driving portion and the worm gear, wherein the gear operating portion comprises: a pusher which operates the unit casing so that the worm gear gear-engages with the rack gear when sliding pressure is applied in one direction; and an elastic member which is connected to the unit casing and elastically biased in a direction in which the worm gear is released from the rack gear. an automatic plunger pressurizing unit which is selectively connected to the plunger in a position adjacent to the plunger and automatically applies pressure to the plunger by means of electrical power; wherein the automatic plunger pressurizing unit comprises: a plunger driving portion which provides the electrical power for applying the pressure to the plunger; a connection gear portion which is selectively connected to or released from the plunger by a gear method; and a gear operating portion which operates the connection gear portion, wherein the connection gear portion comprises: a worm gear which is attached to the plunger driving portion; and a rack gear which is formed on an outer surface of the plunger and gear-engages with the worm gear, wherein the automatic plunger pressurizing unit further comprises a unit casing which integrally supports the plunger driving portion and the worm gear, wherein the gear operating portion comprises: a pusher which operates the unit casing so that the worm gear gear-engages with the rack gear when sliding pressure is applied in one direction; and an elastic member which is connected to the unit casing and elastically biased in a direction in which the worm gear is released from the rack gear.