A ball valve includes an inner valve unit having a ball valve unit and a valve seat in which the ball valve unit is received. A shaft extends from the ball valve which is clamped between two retainers. The valve seat has a tubular shank and a lateral hole is defined radially through the shank. The shaft extends through the valve seat. The housing is made of metal and has a chamber for receiving the inner valve unit. The housing has an inlet and an outlet which is located corresponding to the lateral hole of the valve seat and the central holes of the retainers. The shaft extends through the installation hole of the housing and is connected with a switch which is operated to rotate the ball valve between open and sealed positions. The water does not contact the metallic parts and the structure is simple and easily assembled.