A modification method comprising the steps of: using, as a foundation, an existing axial-flow compressor that has variable stator vanes that are located only at a first stage, constitute a stator vane row and rotate around rotary shafts of the variable stator vanes so as to adjust angles of the variable stator vanes, the shafts extending in a radial direction of the axial-flow compressor; characterized by replacing at least stator vanes included in the existing axial-flow compressor and located at a second stage with other variable stator vanes; connecting a new ring to another stator vane row constituted by the other variable stator vanes so that the ring drives and rotates the other variable stator vanes of the other stator vane row; adding, to the existing axial-flow compressor, a plurality of levers (12a, 12b), a new rotary shaft and a plurality of connecting rods, the plurality of levers (12a, 12b) corresponding to the ring and another ring that is located at the first stage, the new rotary shaft holding the plurality of levers (12a, 12b) so as to enable the levers (12a, 12b) to pivot, extending in an axis of the axial-flow compressor, the connecting rods connecting the plurality of rings to the plurality of levers (12a, 12b) that correspond to the rings, respectively; and connecting a cylinder included in the existing axial-flow compressor to one lever of the plurality of levers (12a, 12b), which is associated with the stator vane row located at the first stage, and the one lever of the plurality of levers (12a, 12b) is connected by the rotary shaft to a further lever of the plurality of levers (12a, 12b) connected to the first ring. using, as a foundation, an existing axial-flow compressor that has variable stator vanes that are located only at a first stage, constitute a stator vane row and rotate around rotary shafts of the variable stator vanes so as to adjust angles of the variable stator vanes, the shafts extending in a radial direction of the axial-flow compressor; characterized by replacing at least stator vanes included in the existing axial-flow compressor and located at a second stage with other variable stator vanes; connecting a new ring to another stator vane row constituted by the other variable stator vanes so that the ring drives and rotates the other variable stator vanes of the other stator vane row; adding, to the existing axial-flow compressor, a plurality of levers (12a, 12b), a new rotary shaft and a plurality of connecting rods, the plurality of levers (12a, 12b) corresponding to the ring and another ring that is located at the first stage, the new rotary shaft holding the plurality of levers (12a, 12b) so as to enable the levers (12a, 12b) to pivot, extending in an axis of the axial-flow compressor, the connecting rods connecting the plurality of rings to the plurality of levers (12a, 12b) that correspond to the rings, respectively; and connecting a cylinder included in the existing axial-flow compressor to one lever of the plurality of levers (12a, 12b), which is associated with the stator vane row located at the first stage, and the one lever of the plurality of levers (12a, 12b) is connected by the rotary shaft to a further lever of the plurality of levers (12a, 12b) connected to the first ring.