A gas turbine casing configured to accommodate a compressor, a combustor, and a turbine, the gas turbine casing comprising: a first casing having a cylindrical shape and having a first flange portion, the first flange portion having a ring shape and being provided at an axial end of the first casing, the first flange portion having a plurality of first connecting holes provided along a circumferential direction of the first flange portion; a plurality of first notches provided on an outer periphery of the first flange portion such that the respective first notches are provided between the first connecting holes; a second casing having a cylindrical shape and having a second flange portion, the second flange portion having a ring shape being provided at an axial end of the second flange portion, the second flange portion having a plurality of second connecting holes provided along a circumferential direction of the second flange portion; a plurality of second notches provided on an outer periphery of the second flange portion such that the respective second notches are provided between the second connecting holes; and a plurality of fastening bolts configured to fasten the first flange portion and the second flange portion with the portions (42,52) being closely attached to each other and with each fastening bolt penetrating the corresponding first connecting hole and the corresponding second connecting hole, characterized in that a first radial direction ratio L/H is set to be from 0.09 to 0.11, where H is a length of each of the first flange portion and the second flange portion in a radial direction, and L is a length of each of the first notch and the second notch in a radial direction. a first casing having a cylindrical shape and having a first flange portion, the first flange portion having a ring shape and being provided at an axial end of the first casing, the first flange portion having a plurality of first connecting holes provided along a circumferential direction of the first flange portion; a plurality of first notches provided on an outer periphery of the first flange portion such that the respective first notches are provided between the first connecting holes; a second casing having a cylindrical shape and having a second flange portion, the second flange portion having a ring shape being provided at an axial end of the second flange portion, the second flange portion having a plurality of second connecting holes provided along a circumferential direction of the second flange portion; a plurality of second notches provided on an outer periphery of the second flange portion such that the respective second notches are provided between the second connecting holes; and a plurality of fastening bolts configured to fasten the first flange portion and the second flange portion with the portions (42,52) being closely attached to each other and with each fastening bolt penetrating the corresponding first connecting hole and the corresponding second connecting hole, characterized in that a first radial direction ratio L/H is set to be from 0.09 to 0.11, where H is a length of each of the first flange portion and the second flange portion in a radial direction, and L is a length of each of the first notch and the second notch in a radial direction.