A gas expander comprising: a casing in which a swirl chamber is formed, a gas to be expanded passing through the swirl chamber; a turbine wheel that is housed in the casing and rotationally driven by the gas acting on the turbine wheel, the gas having passed through the swirl chamber and expanded; and a diffuser that is mounted to the casing in a direction of a rotating shaft of the turbine wheel, and has a flow path through which the gas having passed through the turbine wheel flows in the direction of the rotating shaft of the turbine wheel, wherein the gas expander comprises: a swirl stopper that is provided in the diffuser and faces a downstream front end surface of a boss of the turbine wheel facing the flow path; and a swirl preventing plate or a plurality of swirl preventing plates that circumferentially partition the flow path in the diffuser, wherein the swirl stopper includes a closed swirl stopping surface, that is provided to face the downstream front end surface of the boss with a minute gap between the swirl stopping surface and the downstream front end surface of the boss, the swirl stopper further includes a cylindrical member provided coaxially with the rotating shaft of the turbine wheel, the gas expander being characterised in that D13 × 1.2 ≥ D32 ≥ D13 × 0.95 is satisfied, where D32 is a diameter of the swirl stopping surface of the swirl stopper, and D13 is a diameter of the downstream front end surface of the boss of the turbine wheel. a casing in which a swirl chamber is formed, a gas to be expanded passing through the swirl chamber; a turbine wheel that is housed in the casing and rotationally driven by the gas acting on the turbine wheel, the gas having passed through the swirl chamber and expanded; and a diffuser that is mounted to the casing in a direction of a rotating shaft of the turbine wheel, and has a flow path through which the gas having passed through the turbine wheel flows in the direction of the rotating shaft of the turbine wheel, wherein the gas expander comprises: a swirl stopper that is provided in the diffuser and faces a downstream front end surface of a boss of the turbine wheel facing the flow path; and a swirl preventing plate or a plurality of swirl preventing plates that circumferentially partition the flow path in the diffuser, wherein a swirl stopper that is provided in the diffuser and faces a downstream front end surface of a boss of the turbine wheel facing the flow path; and a swirl preventing plate or a plurality of swirl preventing plates that circumferentially partition the flow path in the diffuser, wherein the swirl stopper includes a closed swirl stopping surface, that is provided to face the downstream front end surface of the boss with a minute gap between the swirl stopping surface and the downstream front end surface of the boss, the swirl stopper further includes a cylindrical member provided coaxially with the rotating shaft of the turbine wheel, the gas expander being characterised in that D13 × 1.2 ≥ D32 ≥ D13 × 0.95 is satisfied, where D32 is a diameter of the swirl stopping surface of the swirl stopper, and D13 is a diameter of the downstream front end surface of the boss of the turbine wheel.