A drug mixing kit for mixing a first content in a liquid form in a first container with a second content in a liquid or powder form in a second container, the drug mixing kit comprising: a syringe forming therein an accommodation space configured to accommodate a liquid; and a drug mixing device detachably coupled to the syringe and including a drug flow path extending from one end configured to allow the liquid in the accommodation space to flow into the drug flow path therethrough to the other end configured to allow the liquid or powder in the second container to flow into the drug flow path therethrough, wherein the drug mixing device includes a body including a container connection part configured to be couplable with the second container and a syringe connection part configured to be connectable with the syringe; and a valve disposed in the drug flow path and configured to open/close the liquid in the drug flow path, characterized in that the syringe includes: a syringe housing forming a space therein; and a plunger configured to change a volume of the accommodation space inside the syringe housing by being inserted into and moving in the syringe housing, the plunger forming therein an introduction space into which the first container is introduced and an inner flow path through which the introduction space and the accommodation space communicate with each other, wherein the plunger includes an inner needle forming therein at least a portion of the inner flow path and protruding into the introduction space; and a backflow prevention part disposed in the inner flow path, and configured to prevent a flow of a liquid from a side of the accommodation space to a side of the introduction space while allowing a flow of the liquid from the side of the introduction space to the side of the accommodation space, and characterized in that the drug mixing device further includes a sealer disposed in a partial flow path (Pa) of the drug flow path extending to a side of the one end with reference to a point at which the valve is disposed in the drug flow path, and the syringe further includes a syringe needle configured to penetrate the sealer and forming therein a flow path to guide a flow of the liquid between the drug flow path and the accommodation space. a syringe forming therein an accommodation space configured to accommodate a liquid; and a drug mixing device detachably coupled to the syringe and including a drug flow path extending from one end configured to allow the liquid in the accommodation space to flow into the drug flow path therethrough to the other end configured to allow the liquid or powder in the second container to flow into the drug flow path therethrough, wherein the drug mixing device includes a body including a container connection part configured to be couplable with the second container and a syringe connection part configured to be connectable with the syringe; and a valve disposed in the drug flow path and configured to open/close the liquid in the drug flow path, characterized in that the syringe includes: a syringe housing forming a space therein; and a plunger configured to change a volume of the accommodation space inside the syringe housing by being inserted into and moving in the syringe housing, the plunger forming therein an introduction space into which the first container is introduced and an inner flow path through which the introduction space and the accommodation space communicate with each other, wherein the plunger includes an inner needle forming therein at least a portion of the inner flow path and protruding into the introduction space; and a backflow prevention part disposed in the inner flow path, and configured to prevent a flow of a liquid from a side of the accommodation space to a side of the introduction space while allowing a flow of the liquid from the side of the introduction space to the side of the accommodation space, and characterized in that the drug mixing device further includes a sealer disposed in a partial flow path (Pa) of the drug flow path extending to a side of the one end with reference to a point at which the valve is disposed in the drug flow path, and a syringe housing forming a space therein; and a plunger configured to change a volume of the accommodation space inside the syringe housing by being inserted into and moving in the syringe housing, the plunger forming therein an introduction space into which the first container is introduced and an inner flow path through which the introduction space and the accommodation space communicate with each other, wherein the plunger includes an inner needle forming therein at least a portion of the inner flow path and protruding into the introduction space; and a backflow prevention part disposed in the inner flow path, and configured to prevent a flow of a liquid from a side of the accommodation space to a side of the introduction space while allowing a flow of the liquid from the side of the introduction space to the side of the accommodation space, and characterized in that the drug mixing device further includes a sealer disposed in a partial flow path (Pa) of the drug flow path extending to a side of the one end with reference to a point at which the valve is disposed in the drug flow path, and