A valve device comprising: a flow passage block in which a flow passage is formed; a diaphragm configured to open and close the flow passage; an actuator configured to push down the diaphragm via a diaphragm holder; a tubular bonnet formed with an internal thread on an inner circumferential surface, the bonnet being configured to join the flow passage block and the actuator; a tubular adjusting screw threaded to the internal thread, the adjusting screw being configured to come into contact with the diaphragm holder to adjust a valve opening position of the diaphragm; and an annular lock nut configured to lock the adjusting screw by being screwed into the internal thread.