Disclosed is a two-port valve in which a fluid passage between two ports is gradually openable without a rapid flow rate, by stepwise lifting a valve member off a valve sent using two pistons, to open the valve in a phased manner. A cylinder portion 2 for driving a valve member 4 has a first piston 24 through which a valve shaft 11 slidably extends, a sleeve 35 movably disposed in a second pressure chamber 27 to limit an operating stroke of the first piston 24, and a second piston 37 slidably disposed in the sleeve 35 and connected to the shaft 11. In an initial phase of opening of the valve, the pistons 24, 37 acting like a unit are together moved to a position at which the first piston 24 contacts the sleeve 35, to slightly open the valve member 4. Thereafter, the valve member 4 is moved by the second piston 37 to its fully open position.