The ball valve may be manually opened and dosed with ease and securely prevent leakage. The ball valve includes a valve body having a fluid inlet passage, fluid outlet passage, and a communicating part provided between these passages. The ball valve includes a ball having a fluid communication passage and preferably is arranged on the communicating part of the valve body for rotating about a reference axis. A substantially cylindrical ball seat which preferably has a desired opening part on its peripheral wall and is arranged between the ball and valve body. The ball valve includes a ball seat pressing part inside the valve body, adjacent to the ball seat and a presser screw screwed into the inner periphery of the valve body. The presser screw preferably presses the ball seat pressing part by adjustment of its tightening force and presses the ball seat pressing part without contacting the packing.