To provide a structurally simple butterfly valve that permits determination of a flow rate when a small quantity of water (fluid) is caused to flow, for example, in a watering operation for a downstream side of piping.

An outer circumferential portion of a valve element 2 has fins 30A and 30B placed in contact with valve casing seats 1A to interrupt fluid passage in a small travel range up to some 20 degrees of the valve element, and the fin 30B situated upstream along fluid passage in the small travel range has a passage 31 for allowing fluid passage. When the valve element 2 is opened over small travel, a small quantity of water is allowed to flow through the passage 31 from an upstream side to a downstream side of a valve casing 1, whereat the flow rate of the small quantity of flowing water is determined from a sectional area of the passage 31.