To provide a seat structure capable of limiting moving amount of a ball, even if fluid pressure acts on the ball, and preventing abnormal rise of torque and generation of operation defects and having superior sealing property, even if the ball valve is of a floating type.

In this ball valve, in which the ball 14 is incorporated rotatably into a valve casing 12 provided with fluid passages 11 and 11 on both sides, valve seat structures 16 in sliding contact with the ball 14 are provided at positions on both sides sandwiching the balls 14 within the valve casing 12 and the communication of both the fluid passages 11 and 11 is opened/closed by the turning operation of the ball 14, the valve seat structure 16 is constituted by an integral structure of a soft seat 17, in sliding contact with the ball 14 and a core 18 formed, by using a material softer than that of the ball 14, and the core 18 is used also as a stopper of the ball 14, when the soft sheet 17 receives compression by the ball 14.