A ball valve includes: a main body having a fluid inlet passage, a fluid outlet passage, and a column-shaped depression communicating with these passages and opening upward; a ball including a fluid communication passage and disposed at the column-shaped depression of the main body rotatably about a reference axis; an operating mechanism configured to cause the ball to rotate about the reference axis; a ball seat having a required opening on a peripheral wall thereof and disposed between the ball and the main body; a ball seat presser configured to press the ball seat; and a pressing screw configured to press the ball seat presser downward. The ball is rotated to achieve communication and cutoff. The ball seat has a substantially frustoconical shape having a bottom surface larger than a bottom of the depression in diameter, and an upper surface slightly smaller than the bottom surface in diameter.