An apparatus for preventing backflow, comprising: a first body comprising a hemispherical chamber with an opened surface and comprising a discharge passage in a lower portion of the chamber; a second body mounted on the opened surface of the first body, the second body comprising a seating part on one surface facing one surface of the first body and comprising an inflow passage connected to the seating part; and a disk-shaped backflow prevention member closely mounted to the seating part between the first body and the second body, the backflow prevention member opening or closing between the seating part and the chamber while moving about one end by a predetermined rotation according to the forward flow of the medical fluid flowing in the inflow passage and the backflow generated in the discharge passage, and wherein the backflow prevention member comprises: a blocking body closely coupled to the seating part; a fixing protrusion part protruding from one side of the blocking body and rotatably seated and fixed between one surface of the first body and one surface of the second body; and a moving protrusion part protruding from the other side of the blocking body and provided between one surface of the first body and one surface of the second body, the moving protrusion part being guided to move in an up-and-down direction between the chamber and the seating part, and wherein between the first body and the second body are formed a fixing part and a moving guide, wherein the fixing part is formed between the first body and the second body so as the fixing protrusion part is rotatably fixed to the fixing part and wherein the moving guide is formed between the first body and the second body and formed at a position facing the fixing part, so as to guide up-and-down movement of the moving protrusion part. a first body comprising a hemispherical chamber with an opened surface and comprising a discharge passage in a lower portion of the chamber; a second body mounted on the opened surface of the first body, the second body comprising a seating part on one surface facing one surface of the first body and comprising an inflow passage connected to the seating part; and a disk-shaped backflow prevention member closely mounted to the seating part between the first body and the second body, the backflow prevention member opening or closing between the seating part and the chamber while moving about one end by a predetermined rotation according to the forward flow of the medical fluid flowing in the inflow passage and the backflow generated in the discharge passage, and wherein the backflow prevention member comprises: a blocking body closely coupled to the seating part; a fixing protrusion part protruding from one side of the blocking body and rotatably seated and fixed between one surface of the first body and one surface of the second body; and a moving protrusion part protruding from the other side of the blocking body and provided between one surface of the first body and one surface of the second body, the moving protrusion part being guided to move in an up-and-down direction between the chamber and the seating part, and wherein between the first body and the second body are formed a fixing part and a moving guide, wherein the fixing part is formed between the first body and the second body so as the fixing protrusion part is rotatably fixed to the fixing part and wherein the moving guide is formed between the first body and the second body and formed at a position facing the fixing part, so as to guide up-and-down movement of the moving protrusion part. a blocking body closely coupled to the seating part; a fixing protrusion part protruding from one side of the blocking body and rotatably seated and fixed between one surface of the first body and one surface of the second body; and a moving protrusion part protruding from the other side of the blocking body and provided between one surface of the first body and one surface of the second body, the moving protrusion part being guided to move in an up-and-down direction between the chamber and the seating part, and wherein between the first body and the second body are formed a fixing part and a moving guide, wherein the fixing part is formed between the first body and the second body so as the fixing protrusion part is rotatably fixed to the fixing part and wherein the moving guide is formed between the first body and the second body and formed at a position facing the fixing part, so as to guide up-and-down movement of the moving protrusion part.