Abstract: A gate valve has a transfer chamber-side valve element and a process chamber-side valve element connected to a single shaft actuated with a single three-position air cylinder to selectively close a process chamber opening and a transfer chamber opening or fully open both the openings to allow an object to pass therethrough. A lever member is secured to the single shaft. A roller supported by the lever member is kept in engagement with a cam groove of a cam member. A cam support member for supporting the cam member is connected to the piston rod of the three-position air cylinder. A spring is interposed between the lever member and the cam support member. When the piston rod is in the first (lower extremity) position, both the transfer chamber-side valve element and the process chamber-side valve element fully open the transfer chamber opening and the process chamber opening, respectively.