A structure for a gas turbine casing, capable of preventing gas from leaking when the structure is subjected to pressure. A structure for a gas turbine casing, divided by a horizontal plane at flange sections into two halves comprising an upper-half casing (10) and a lower-half casing (11). The structure is provided with bolts (12) which are disposed in the inner surfaces of the upper-half casing (10) and the lower-half casing (11) so as to bridge therebetween, upper nuts (13) which are disposed in the inner surface of the upper-half casing (10) and attached to the bolts (12), and lower nuts (14) which are disposed in the inner surface of the lower-half casing (11) and attached to the bolts (12).