An air separator for a gas turbine, in which cracks are prevented from occurring at a flange portion of the air separator due to fretting fatigue. The air separator (20) has a cylindrical split structure formed by two separator members (20-1 and 20-2), of which one of the separators (20-1) is mounted on a rotor (1) whereas the other separator member (20-2) is mounted on a disc portion (7) on a side of a moving blade (2) by bolts (28) extending through bolt holes (23) formed in a flange portion (22). Cooling air from a compressor enters a space (6) from a duct (5) and passes to a passage (32) from a clearance (33) so that it is fed to air feed holes (43) and radial holes (44) of the disc portion (7). Alternatively, air holes (50) are provided in the flange portion (22) in the form of circumferential slots for covering a plurality of radial holes to feed the cooling air homogeneously so that a prior art air separator of an existing plant can be replaced.