a valve device comprises a valve body having an inflow passage and an outflow passage , a valve seat fixed to the valve body at a position between the passages , a valve element for seating against and separating from the valve seat , a valve stem movable together with the valve element toward and away from the valve seat , a stem holding member secured at a portion thereof to the valve body in the body interior and holding the valve stem at a screw engagement part therewith where a surface at an extremity of the portion of the stem holding member is in abutting contact with a surface of the main body thereby to form a first metal - contact seal , a bonnet which is detachably fixed to the valve body with a sealing member interposed therebetween , and through which the valve stem is rotatably fitted , where the bonnet covers the remainder portion of the stem holding member , and a mechanism for forming a second metal - contact seal at a stem holding part of the stem holding member when the valve is in the fully opened state thereof .