An axial flow expander for expanding gas flowed in a direction along an axis thereof, comprising: a cylindrical shaped outer casing centered around the axis, and provided with a suction port through which the gas is suctioned into the outer casing and a discharge port formed in both ends of the outer casing with respect to the suction port, wherein the gas is discharged from the outer casing through both ends thereof; a cylindrical shaped inner casing centered around the axis, fixed inside the outer casing, and inside which a gas passage is formed, wherein the inner casing is provided with a first communication portion which communicates the gas passage with the suction port and a second communication portion which communicates the gas passage with the discharge port; a rotor shaft accommodated inside the inner casing, and of which an axial center is aligned with the axis so as to extend in the direction along the axis; a bearing fixed to the inner casing, and configured to bear the rotor shaft so as to be available to rotate around the axis with respect to the inner casing; a plurality of stator vanes fixed to the inner casing so as to protrude inward from an inner periphery of the inner casing, and 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 bearing, the stator vanes, and the moving blades are integrally assembled, and the assembled members are inserted into the outer casing in the direction along the axis, wherein the inner casing comprises: an inner casing body having the stator vanes, in which the first and second communication portions are formed; and first and second head members (25A, 25B) fixed to one end and the other end of the inner casing body in the direction of the axis, and to which the bearing is fixed, wherein the inner casing body comprises: a main body placed at the central position in the direction of the axis; and a connection portion having a first discharging portion casing placed between the main body and the first head member and a second discharging portion casing placed between the main body and the second head member. a cylindrical shaped outer casing centered around the axis, and provided with a suction port through which the gas is suctioned into the outer casing and a discharge port formed in both ends of the outer casing with respect to the suction port, wherein the gas is discharged from the outer casing through both ends thereof; a cylindrical shaped inner casing centered around the axis, fixed inside the outer casing, and inside which a gas passage is formed, wherein the inner casing is provided with a first communication portion which communicates the gas passage with the suction port and a second communication portion which communicates the gas passage with the discharge port; a rotor shaft accommodated inside the inner casing, and of which an axial center is aligned with the axis so as to extend in the direction along the axis; a bearing fixed to the inner casing, and configured to bear the rotor shaft so as to be available to rotate around the axis with respect to the inner casing; a plurality of stator vanes fixed to the inner casing so as to protrude inward from an inner periphery of the inner casing, and 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 bearing, the stator vanes, and the moving blades are integrally assembled, and the assembled members are inserted into the outer casing in the direction along the axis, wherein the inner casing comprises: an inner casing body having the stator vanes, in which the first and second communication portions are formed; and first and second head members (25A, 25B) fixed to one end and the other end of the inner casing body in the direction of the axis, and to which the bearing is fixed, an inner casing body having the stator vanes, in which the first and second communication portions are formed; and first and second head members (25A, 25B) fixed to one end and the other end of the inner casing body in the direction of the axis, and to which the bearing is fixed, wherein the inner casing body comprises: a main body placed at the central position in the direction of the axis; and a connection portion having a first discharging portion casing placed between the main body and the first head member and a second discharging portion casing placed between the main body and the second head member. a main body placed at the central position in the direction of the axis; and a connection portion having a first discharging portion casing placed between the main body and the first head member and a second discharging portion casing placed between the main body and the second head member.