An object is to provide a gate valve capable of preventing operational deficiencies caused by particulate materials accumulating in a valve box, thus providing superior sealing properties and high durability, having a simple structure, and allowing inspection and maintenance to be performed easily. A gate valve installed in a transport line for transporting fluid containing particulate material at a coal gasification plant includes a valve box having an inlet portion and an outlet portion in which a channel constituting a part of the transport line is formed; a valve unit, provided between the inlet portion and the outlet portion, which extends in an opening-and-closing direction orthogonal to an axial direction of the channel, and which is movable in the opening-and-closing direction; a wedge plate provided between the valve unit and the inlet portion so as to extend in the opening-and-closing direction; and a guide plate provided between the valve unit and the outlet portion so as to extend in the opening-and-closing direction. Areas between the valve unit and the wedge plate and between and the valve unit and guide plate are sealed by means of surface contact.