In a steam turbine plant in which a turbine casing containing a turbine is constituted of an upper-half casing and a lower-half casing, nozzles are provided to the upper-half casing and a steam supplied through main steam piping lines are deliveried into the upper-half casing. Main steam pipes for supplying a steam from respective main valves to the respective nozzles are each formed so as to be dividable in a position out of the installation area of the upper-half casing.