A valve device that can be activated by pilot air of comparatively low pressure though the size of such valve device is small and provides a valve device that is excellent in durability. The valve device comprises a pilot port which pilot air is supplied to activate a piston part, and a biasing means to press the piston part in the opposite direction to supply and discharge the pilot air, thus controlling the opening and closing of the valve. A first magnetic member is provided on a valve rod; a second magnetic member is provided on a housing. Magnetic attraction is generated between both magnetic members to activate the piston part in the first direction, thus increasing such magnetic attraction by the approach of both magnetic members. An arrangement-relation facing each magnetic member through an air gap is set even when making its closest approach.