Abstract: A ball valve is provided with a valve case forming an internal space that is centrally extending to opposite inlet and outlet sides. A ball valve element is disposed within the interior space and has a bore extending therethrough, a valve seat rotatably supports the ball valve element and a valve stem is fixed to one side of the ball valve element and has an operating lever at one end of the valve stem projecting therefrom outside the valve case. A flow adjusting internal valve assembly means is provided in the bore which comprises an internal cylindrical valve casing in the bore with an internal valve seat portion, and an axially movable internal valve element disposed within the internal cylindrical valve casing. The internal cylindrical valve casing comprises an internally threaded large-diameter portion at its inlet side, a small-diameter guide bore portion at its outlet side and an intermediate portion positioned therebetween, having a tapered inner surface providing the internal valve seat portion.