A drug injection device equipped with a backflow prevention valve, the drug injection device comprising: an upper housing in which at least one surface is sealed by a pressure partition and providing a floodlight lens at the sealed surface; a lower housing, in which at least one surface is sealed by a drug partition, comprising an injection nozzle having an injection path formed at a lower portion; a compartment provided between the upper housing and the lower housing to partition a space filled with a liquid for generating pressure at the upper housing and a space filled with a drug at the lower housing; a backflow prevention valve provided with a slit through which the drug passes at a lower end and allowing the drug to move only toward the injection nozzle, and a drug supplement formed through the drug partition, and the drug is supplied to a space on the side of the lower housing through the drug supplement; wherein, when the compartment transmits pressure to the space filled with the drug by expansion of the liquid for generating pressure, the slit opens and the drug is injected through the injection nozzle, and when the pressure in the space filled with the drug is lowered, the slit is closed by a restoring force of the backflow prevention valve, so that fluid is prevented from being sucked from the outside, wherein the backflow prevention valve comprises an elastic material, and the slit of the backflow prevention valve remains closed while the drug is filled inside the lower housing, and wherein an upper side of the backflow prevention valve is provided in a lower side of the lower housing to be in close contact with the lower housing, and the backflow prevention valve has an open circular inlet on an upper side, and the cross section of the backflow prevention valve becomes narrower toward a lower side to form a linear shape. an upper housing in which at least one surface is sealed by a pressure partition and providing a floodlight lens at the sealed surface; a lower housing, in which at least one surface is sealed by a drug partition, comprising an injection nozzle having an injection path formed at a lower portion; a compartment provided between the upper housing and the lower housing to partition a space filled with a liquid for generating pressure at the upper housing and a space filled with a drug at the lower housing; a backflow prevention valve provided with a slit through which the drug passes at a lower end and allowing the drug to move only toward the injection nozzle, and a drug supplement formed through the drug partition, and the drug is supplied to a space on the side of the lower housing through the drug supplement; wherein, when the compartment transmits pressure to the space filled with the drug by expansion of the liquid for generating pressure, the slit opens and the drug is injected through the injection nozzle, and when the pressure in the space filled with the drug is lowered, the slit is closed by a restoring force of the backflow prevention valve, so that fluid is prevented from being sucked from the outside, wherein the backflow prevention valve comprises an elastic material, and the slit of the backflow prevention valve remains closed while the drug is filled inside the lower housing, and wherein an upper side of the backflow prevention valve is provided in a lower side of the lower housing to be in close contact with the lower housing, and the backflow prevention valve has an open circular inlet on an upper side, and the cross section of the backflow prevention valve becomes narrower toward a lower side to form a linear shape.