A wedge-shaped gate valve is provided which, while reducing an unwanted thickened portion to improve light weight and castability, ensures strength to include a valve box with high stiffness, prevents deformation of the valve seat surface of the valve box at the time of valve closing, and ensures surface-pressure sealability with the valve disk to reliably prevent leakage. A branching part 12 is formed to be open in a direction orthogonal to an inner flow path 10 of a valve box 2 having a coupling part 11 on each of both sides, and a valve disk 4 ascends and descends from this branching part 12 via a valve stem 3 to open and close an annular valve seat surface 21 in the valve box 2. At a boundary between the coupling part 11 and the branching part 12, a reinforcing thickened part 20 having a substantially cylindrical shape or a substantially cone shape or substantially truncated cone shape with a diameter gradually reducing from this branching part 12 toward the coupling part 11 side is provided, and this thickened part 20 is formed by taking, as an outer shape, a cylinder, a cone, or a truncated cone with an axis P2 which is eccentric from a center line P1 of the flow path to the branching part 12 side and is parallel to or crossing the center line P1 of the flow path taken as a rotation axis.