Conventionally, an opening/closing valve is mounted to a passage through which liquid flows for regulating the flow of the liquid. Such opening/closing valves have various structures, and for instance, there is a ball type valve with a spherical valve body as described in Japanese Utility Model Registration No. 2546566. This ball valve has a spherical valve body, a stem (operating shaft), for a rotational operation of the valve body, connected to the valve body, and a passage provided through the center of the valve body by boring in the direction perpendicular to the stem. The passage of the valve body can be switched between an open state where the passage of the valve body is corresponding to a flow passageway formed in a main body of the ball valve, and a closed state where the passage is perpendicular to the flow passageway.
Such a ball valve is provided with sealing members, which adhere to the outer peripheral surface of the valve body that is to be rotationally operated, at both of the upstream and the downstream sides of the valve body. In the closed state, high sealing efficiency is achieved by sealing the valve body with both of the seal members. Incidentally, such a ball valve as described above has a valve chamber for accommodating the spherical shaped valve body in the main body, and there is a gap between the valve body and the valve chamber. When the valve body is in the closed state, the passage formed in the valve body communicates with a space inside the valve chamber, and the space formed by the valve chamber and the passage is filled with the fluid that has communicated.