PURPOSE: To eliminate the need for power and operation by providing a mechanism adopting the principle of siphon for the gate door of flood way.

CONSTITUTION: A siphon 3 whose one end is opened in the main water channel 1 and the other end is opened in the flood way, a siphon breaker 7 whose one end is opened at the top of the siphon 3 and the other end is opened in a water level regulating tube 6 opening on the downstream side of the main water channel 1, and an air suction tube 8 whose one end is opened in the top of the water level regulating tube 6 and the other end is opened on the downstream side of the flood way 2 are provided. When the water level reaches a design water level (a), the siphon 3 begins to operate and overflowing is started from the siphon 3. When the water level rises further, the serrated opening 7a of the siphon breaker 7 sinks under water. Since the supply of air to the siphon 3 is limited, excess water is quickly discharged by the action of the siphon 3.