A gas expander comprising: a scroll casing configured to send gas inside; an impeller accommodated in the scroll casing and configured to be rotationally driven to a first side in a circumferential direction around a central axis by the gas flowing while expanding from an outside to an inside in a radial direction (Dr); a diffuser disposed on a first side in an axial direction in which the central axis extends with respect to the scroll casing and forming a flow path of the gas discharged from the impeller to the first side in the axial direction and swirling to the first side in the circumferential direction; and a vortex preventer disposed in the diffuser, wherein the impeller is configured to generate a vortex flow which swirls the gas discharged to the first side in the axial direction to the first side in the circumferential direction, the vortex preventer includes a profile rectifying blade portion, which is curved to a second side in the circumferential direction from the first side toward a second side in the axial direction, at least in a portion of the second side in the axial direction, and characterized by a plate rectifying blade portion continuously formed to the first side in the axial direction from the profile rectifying blade portion and having a flat plate shape extending in the axial direction, the plate rectifying blade portion is formed so as to overlap the central axis over the entire area of the axial direction when viewed in the radial direction (Dr), and the profile rectifying blade portion is curved so as to move away from a position of the plate rectifying blade portion in the circumferential direction toward a position away from the plate rectifying blade portion toward the second side in the circumferential direction as it approaches the impeller from the second straightening blade portion and so as to approach the impeller from the plate rectifying blade portion in the axial direction, when viewed in the radial direction. a scroll casing configured to send gas inside; an impeller accommodated in the scroll casing and configured to be rotationally driven to a first side in a circumferential direction around a central axis by the gas flowing while expanding from an outside to an inside in a radial direction (Dr); a diffuser disposed on a first side in an axial direction in which the central axis extends with respect to the scroll casing and forming a flow path of the gas discharged from the impeller to the first side in the axial direction and swirling to the first side in the circumferential direction; and a vortex preventer disposed in the diffuser, wherein the impeller is configured to generate a vortex flow which swirls the gas discharged to the first side in the axial direction to the first side in the circumferential direction, the vortex preventer includes a profile rectifying blade portion, which is curved to a second side in the circumferential direction from the first side toward a second side in the axial direction, at least in a portion of the second side in the axial direction, and characterized by a plate rectifying blade portion continuously formed to the first side in the axial direction from the profile rectifying blade portion and having a flat plate shape extending in the axial direction, the plate rectifying blade portion is formed so as to overlap the central axis over the entire area of the axial direction when viewed in the radial direction (Dr), and the profile rectifying blade portion is curved so as to move away from a position of the plate rectifying blade portion in the circumferential direction toward a position away from the plate rectifying blade portion toward the second side in the circumferential direction as it approaches the impeller from the second straightening blade portion and so as to approach the impeller from the plate rectifying blade portion in the axial direction, when viewed in the radial direction.