To provide a float type auxiliary valve-incorporated butterfly valve that can avoid a water hammer by virtue of gradual constriction of the sectional area of water passage even when a valve element is turned from a small travel range to a fully closing position.

While a main valve element 20 and an auxiliary valve element 21 constituting a valve element 2 are in a fully opening state, a rise in the water level of a water tank 5 in a first effective water level range L raises a main valve element-side float 90 to close the main valve element 20, and a rise in the water level of the water tank 5 from the first effective water level range L to a second effective water level range H raises an auxiliary valve element-side float 91 to close the auxiliary valve element 21. The turning movement of the auxiliary valve element 21 whose valve travel causes a small change in the flow rate of water offers full closing from a small travel range to thus gradually constrict the sectional area of water passage and prevent a sudden stop of the line water flow.