A disk assembly comprising: a first disk engaged with a compressor section of a gas turbine; a second disk engaged with a turbine section of the gas turbine; and a third disk disposed between the first and second disks (1210, 1230), and transferring torque applied to the second disk to the first disk, wherein each of the first to third disks (1210, 1220, 1230) comprises a disk body (1214, 1224, 1234) through which a tie bolt of the gas turbine passes, and an outer rim (1212, 1222, 1232) formed on an outer circumference of the disk body (1214, 1224, 1234) and coupling the disks to each other such that the disks cannot rotate relative to each other, and wherein the disk body of the third disk has first and second spaces formed at respective sides of the third disk, and the first and second spaces communicate with each other through a through-hole formed in the disk body of the third disk, wherein the third disk has an inner rim formed at an inner side thereof in a radial direction, the inner rim being extended in a longitudinal direction of the tie bolt, wherein each of the first and second disks (1210, 1230) comprises an inner rim (1215, 1235) which is in contact with the inner rim of the third disk, characterized in that the third disk has a plurality of legs formed on an inner circumferential surface thereof, the plurality of legs being in contact with an outer circumferential surface of the tie bolt, wherein the legs are radially arranged around a center of the third disk, and a third space is formed by the inner rims (1215, 1225, 1235), the outer rims (1212, 1222, 1232), and the disk bodies (1214, 1224, 1234), wherein the third space is disposed at the outer side in the radial direction from the first space and the second space and does not communicate with the first space and the second space. a first disk engaged with a compressor section of a gas turbine; a second disk engaged with a turbine section of the gas turbine; and a third disk disposed between the first and second disks (1210, 1230), and transferring torque applied to the second disk to the first disk, wherein each of the first to third disks (1210, 1220, 1230) comprises a disk body (1214, 1224, 1234) through which a tie bolt of the gas turbine passes, and an outer rim (1212, 1222, 1232) formed on an outer circumference of the disk body (1214, 1224, 1234) and coupling the disks to each other such that the disks cannot rotate relative to each other, and wherein the disk body of the third disk has first and second spaces formed at respective sides of the third disk, and the first and second spaces communicate with each other through a through-hole formed in the disk body of the third disk, wherein the third disk has an inner rim formed at an inner side thereof in a radial direction, the inner rim being extended in a longitudinal direction of the tie bolt, wherein each of the first and second disks (1210, 1230) comprises an inner rim (1215, 1235) which is in contact with the inner rim of the third disk, characterized in that the third disk has a plurality of legs formed on an inner circumferential surface thereof, the plurality of legs being in contact with an outer circumferential surface of the tie bolt, wherein the legs are radially arranged around a center of the third disk, and a third space is formed by the inner rims (1215, 1225, 1235), the outer rims (1212, 1222, 1232), and the disk bodies (1214, 1224, 1234), wherein the third space is disposed at the outer side in the radial direction from the first space and the second space and does not communicate with the first space and the second space.