A drain treatment system capable of achieving balance of pressure between a facility apparatus and a drain treatment apparatus with a simple structure is provided.A bottom part of a heat, exchanger 2 is connected with a top part of a drain tank 3 by a drain discharge pipe 4. The drain discharge pipe 4 has a balance pipe-connecting tube 5. The balance pipe-connecting tube 5 comprises: a balance pipe-connecting part 6; a communicating tube-connecting part 7; and a communicating passage 8 which connects both the parts therein. The top part of the drain tank 3 is connected with the balance pipe-connecting part 6 by a balance pipe 9. Further, the communicating tube-connecting part 7 is connected with a lower part of the heat exchanger 2 by a communicating tube 10 which passes through the drain discharge pipe 4. If a drain level of the heat exchanger 2 is below an open end of the communicating tube 8 on the side of the heat exchanger 2, the balance of pressure is achieved between the heat exchanger 2 and the drain tank 3 by the balance pipe 9, the communicating passage 8, and the communicating tube 10, allowing the drain from the heat exchanger 2 to be smoothly discharged into the drain tank 3 through the drain discharge pipe 4.