Tapered thread for gas turbine shroud

1. A gas turbine comprising a compressor adiabatically compressing air taken in from the atmosphere, as operation fluid, a combustor mixing and combusting the compressed air supplied from the compressor and fuel, to thereby produce high temperature-high pressure gas, and a turbine in which moving vanes and stationary vanes are alternately arranged, the turbine being adapted to generate rotary power at the time of expansion of the combusted gas introduced from the combustor; wherein

shroud segments are provided at upper portions of the moving vanes, the shroud segments are axially and radially fixed to a turbine casing and circumferentially fixed to the turbine casing by shroud lock pins that are inserted into shroud lock pin holes formed in the turbine casing, and covers are provided to the shroud lock pin holes by tapered threads, wherein

each of the tapered threads is a taper-shaped external thread that is threadedly engaged with a taper-shaped internal thread formed in a member that is to be fastened, wherein

a center hole is formed in a center of a bottom surface of the taper-shaped external thread, and a depth of the center hole is not more than half of an axial length of the taper-shaped external thread, wherein

a diameter and the depth of the center hole are set in such a manner to suppress an increase of a contact surface pressure at a portion extending to a tip end side from a half position of an axial length of the external thread.