A gate valve in which a gate opening formed in a valve casing is opened and closed by a valve plate disposed in the valve casing, the gate valve comprising: the valve plate; a valve shaft that has an end portion attached to the valve plate and another end portion extending from the valve casing and that is supported in such a manner as to be movable with respect to the valve casing; an air cylinder that is disposed outside the valve casing and that includes a drive rod; and a valve moving mechanism that causes the valve plate to move along with expanding and contracting movements of the drive rod of the air cylinder via the valve shaft such that the valve plate reciprocates between a hermetically closed position at which the gate opening is hermetically closed by the valve plate and a full open position at which the gate opening is fully opened, wherein the valve moving mechanism includes a first block that is fixed to the drive rod, a second block that is fixed to the valve shaft extending from the valve casing, and a connecting member that connects the second block to the first block such that the second block is movable relative to the first block, wherein the air cylinder includes a cylinder housing that supports the drive rod such that the drive rod is capable of expanding and contracting, wherein the cylinder housing is provided with a locking mechanism for locking the valve plate that has moved to the hermetically closed position, wherein the locking mechanism includes a lock piston that is disposed in a facing wall of the cylinder housing, the facing wall facing the first block, so as to be capable of advancing and retreating with respect to the first block, characterized in that the cylinder housing is further provided with a lock detection unit that is capable of determining whether the valve plate is in a state of being locked by the locking mechanism, wherein the first block has an engagement hole that is formed such that the lock piston that has advanced toward the first block is engaged in the engagement hole in a state in which the valve plate has moved to the hermetically closed position, and wherein the lock detection unit is capable of detecting whether the lock piston is in a state of being engaged in the engagement hole. the valve plate; a valve shaft that has an end portion attached to the valve plate and another end portion extending from the valve casing and that is supported in such a manner as to be movable with respect to the valve casing; an air cylinder that is disposed outside the valve casing and that includes a drive rod; and a valve moving mechanism that causes the valve plate to move along with expanding and contracting movements of the drive rod of the air cylinder via the valve shaft such that the valve plate reciprocates between a hermetically closed position at which the gate opening is hermetically closed by the valve plate and a full open position at which the gate opening is fully opened, wherein the valve moving mechanism includes a first block that is fixed to the drive rod, a second block that is fixed to the valve shaft extending from the valve casing, and a connecting member that connects the second block to the first block such that the second block is movable relative to the first block, a first block that is fixed to the drive rod, a second block that is fixed to the valve shaft extending from the valve casing, and a connecting member that connects the second block to the first block such that the second block is movable relative to the first block, wherein the air cylinder includes a cylinder housing that supports the drive rod such that the drive rod is capable of expanding and contracting, wherein the cylinder housing is provided with a locking mechanism for locking the valve plate that has moved to the hermetically closed position, wherein the locking mechanism includes a lock piston that is disposed in a facing wall of the cylinder housing, the facing wall facing the first block, so as to be capable of advancing and retreating with respect to the first block, characterized in that the cylinder housing is further provided with a lock detection unit that is capable of determining whether the valve plate is in a state of being locked by the locking mechanism, wherein the first block has an engagement hole that is formed such that the lock piston that has advanced toward the first block is engaged in the engagement hole in a state in which the valve plate has moved to the hermetically closed position, and wherein the lock detection unit is capable of detecting whether the lock piston is in a state of being engaged in the engagement hole.