BACKFLOW PREVENTION DEVICE

PROBLEM TO BE SOLVED: To provide a backflow prevention device that can prevent backflow generated by negative pressure from a primary side in addition to reverse pressure from a secondary side.SOLUTION: The backflow prevention device includes: a primary pressure chamber 12 and a fluid passing chamber 13 partitioned by a flexibly deformable diaphragm 16; a drain chamber 14 communicated with the fluid passing chamber 13, which has an outflow port 21 and a drain port 22 for circulating backflow water from the secondary side; an intake chamber 15 communicated with the fluid passing chamber 13, which has an intake port 23 for sucking outside air; and an opening/closing means V1 which closes the drain port 22 and the intake port 23 when the pressure in the primary side is higher than that in the secondary side and opens the drain port 22 and the intake port 23 when the pressure in the secondary side is higher than that in the primary side and closes communication between the fluid passing chamber 13, the drain chamber 14 and the intake chamber 15.