An intake air cooling system 100 for a gas turbine 18 is provided with: an intake duct 12 for leading intake air taken in from an intake-air inlet 22 to a compressor 14 of the gas turbine; a cooling part provided in the intake duct and configured to cool the intake air by heat exchange with a cooling medium which is introduced from an outside; a protruding step part 13 formed in a convex shape protruding from bottom surfaces 12a1, 12a2 of the intake duct disposed on a downstream side of the cooling part; and at least one drain hole 110 formed in the bottom surfaces 12a1, 12a2 of the intake duct disposed on the downstream side of the cooling part so as to discharge drain water, generated on a surface of the cooling part and dropping from the surface, to an outside of the intake duct.