To provide a valve device capable of easily and precisely adjusting a flow rate of a flowing fluid, and an actuator used in the valve device. The problem is solved by an actuator including pistons housed in a housing, a supply port of compressed air provided to a casing constituting the housing, and an adjustment member that is provided to a position separated from the supply port and regulates an upper limit position of possible movement of the piston that comes into contact with the adjustment member. The piston comes into contact with the adjustment member and is positioned in the upper limit position when compressed air is supplied through the supply port. The adjustment member s provided so as to allow adjustment of the upper limit position from outside the housing.