A gas expander includes a scroll casing, an impeller accommodated in the scroll casing and 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, a diffuser disposed on a first side in an axial direction 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 provided in the diffuser, in which the vortex preventer includes a profile rectifying blade portion, which is curved or inclined 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.