An apparatus for preventing backflow. The apparatus includes a housing; a trap flow part forming a flow passage for a medical fluid, a discharge part provided on an lower portion of the housing and to receive the medical fluid from the trap flow part and to discharge the medical fluid; a backflow prevention chamber part provided between the trap flow part and the discharge part; and a plate-shaped backflow prevention member provided on one surface of the inner side of the backflow prevention chamber part connected to the trap flow part and in close contact with a periphery of the outlet opening, the plate-shaped backflow prevention member being provided to open or close the opening depending on forward flow and the backflow of the medical fluid.