A system for decreasing a temperature of a fluid, comprising an axial flow expander for expanding gas flowed in a direction along an axis thereof, comprising: an outer casing centered around the axis, wherein the outer casing is a unified structure having an inlet port and two outlet ports located at an opposite side of the inlet port with respect to the axis so as to protrude in a radial direction of the outer casing; an inner casing centered around the axis, fixed inside the outer casing, wherein the inner casing comprises: an inner casing body comprising a main body having a plurality of stator vanes; a head member fixed to both ends of the inner casing body in the direction of the axis, and to which a plurality of bearings is fixed, wherein the inner casing body comprises a connection portion fixed to both ends of the main body in the direction of the axis, to which the head member is fixed; and a gas passage configured to allow gas to enter the inner casing from the inlet port of the outer casing and exit the inner casing to the outlet ports of the outer casing; a rotor shaft accommodated inside the inner casing, and of which an axial center is aligned with the axis, wherein the axial flow expander is configured to allow fluid to flow in opposing directions along the rotor shaft; the plurality of bearings fixed to the inner casing, wherein the plurality of bearings is configured to allow the rotor shaft to rotate around the axis with respect to the inner casing; the plurality of stator vanes fixed to the inner casing so as to protrude inward from an inner surface of the inner casing, wherein the plurality of stator vanes is arranged inside the gas passage so as to be separated at intervals in the direction of the axis; and a plurality of moving blades fixed to the rotor shaft so as to protrude outward from the rotor shaft, and arranged inside the gas passage so as to be alternating with the plurality of stator vanes; wherein the inner casing, the rotor shaft, the plurality of bearings, the plurality of stator vanes, and the plurality of moving blades are integrally assembled members, and the assembled members are inserted into the outer casing in the direction along the axis. an outer casing centered around the axis, wherein the outer casing is a unified structure having an inlet port and two outlet ports located at an opposite side of the inlet port with respect to the axis so as to protrude in a radial direction of the outer casing; an inner casing centered around the axis, fixed inside the outer casing, wherein the inner casing comprises: an inner casing body comprising a main body having a plurality of stator vanes; a head member fixed to both ends of the inner casing body in the direction of the axis, and to which a plurality of bearings is fixed, wherein the inner casing body comprises a connection portion fixed to both ends of the main body in the direction of the axis, to which the head member is fixed; and a gas passage configured to allow gas to enter the inner casing from the inlet port of the outer casing and exit the inner casing to the outlet ports of the outer casing; an inner casing body comprising a main body having a plurality of stator vanes; a head member fixed to both ends of the inner casing body in the direction of the axis, and to which a plurality of bearings is fixed, wherein the inner casing body comprises a connection portion fixed to both ends of the main body in the direction of the axis, to which the head member is fixed; and a gas passage configured to allow gas to enter the inner casing from the inlet port of the outer casing and exit the inner casing to the outlet ports of the outer casing; a rotor shaft accommodated inside the inner casing, and of which an axial center is aligned with the axis, wherein the axial flow expander is configured to allow fluid to flow in opposing directions along the rotor shaft; the plurality of bearings fixed to the inner casing, wherein the plurality of bearings is configured to allow the rotor shaft to rotate around the axis with respect to the inner casing; the plurality of stator vanes fixed to the inner casing so as to protrude inward from an inner surface of the inner casing, wherein the plurality of stator vanes is arranged inside the gas passage so as to be separated at intervals in the direction of the axis; and a plurality of moving blades fixed to the rotor shaft so as to protrude outward from the rotor shaft, and arranged inside the gas passage so as to be alternating with the plurality of stator vanes; wherein the inner casing, the rotor shaft, the plurality of bearings, the plurality of stator vanes, and the plurality of moving blades are integrally assembled members, and the assembled members are inserted into the outer casing in the direction along the axis.