A method of assembling a turbine including a casing formed by fastening together a casing upper half part (12,22) and a casing lower half part (11,21) by bolts, a stationary part accommodated in the casing, and a rotor accommodated in the casing and configured to rotate with respect to the stationary part, the method comprising the steps of: gaining measurement data on a configuration of the casing upper half part (12,22) not fastened to the casing lower half part (11,21); gaining measurement data on a configuration of the casing lower half part (11,21) in an open state in which the casing upper half part (12,22) and the rotor are removed and in which the stationary part is mounted; comparing measurement data on the configuration of the casing upper half part (12,22) and the casing lower half part (11,21) with simulation data on the configuration of the casing upper half part (12,22) and the casing lower half part (11,21) previously obtained through numerical analysis of a turbine model simulating characteristics of the turbine to select simulation data closest to the measurement data with respect to the configuration of the casing upper half part (12,22) and the casing lower half part (11,21); calculating, based on the selected simulation data, a change amount of the configuration of the casing upper half part (12,22) and the casing lower half part (11,21) when the casing upper half part (12,22) is fastened to the casing lower half part (11,21) in the open state by the bolts; and adjusting an installation position of the stationary part inside the casing taking into account the calculated change amount to assemble the turbine, characterized in that the numerical analysis is creep deformation analysis based on a finite-element method, and values of parameters including a temperature of a working fluid supplied to the turbine, a heat transfer rate between the working fluid and the casing, and an operation time of the turbine are varied using the design condition of the turbine as a reference, and are input to the turbine model. gaining measurement data on a configuration of the casing upper half part (12,22) not fastened to the casing lower half part (11,21); gaining measurement data on a configuration of the casing lower half part (11,21) in an open state in which the casing upper half part (12,22) and the rotor are removed and in which the stationary part is mounted; comparing measurement data on the configuration of the casing upper half part (12,22) and the casing lower half part (11,21) with simulation data on the configuration of the casing upper half part (12,22) and the casing lower half part (11,21) previously obtained through numerical analysis of a turbine model simulating characteristics of the turbine to select simulation data closest to the measurement data with respect to the configuration of the casing upper half part (12,22) and the casing lower half part (11,21); calculating, based on the selected simulation data, a change amount of the configuration of the casing upper half part (12,22) and the casing lower half part (11,21) when the casing upper half part (12,22) is fastened to the casing lower half part (11,21) in the open state by the bolts; and adjusting an installation position of the stationary part inside the casing taking into account the calculated change amount to assemble the turbine, characterized in that the numerical analysis is creep deformation analysis based on a finite-element method, and values of parameters including a temperature of a working fluid supplied to the turbine, a heat transfer rate between the working fluid and the casing, and an operation time of the turbine are varied using the design condition of the turbine as a reference, and are input to the turbine model.