1. A gas turbine performance analysis system comprising: a performance computation module for receiving each of actual measured values of an inlet air temperature introduced to a compressor, a compressor pressure ratio and a fuel flow rate supplied to a gas turbine in the actual gas turbine and calculating and outputting values of the gas turbine's power output and exhaust temperature exhausted from the gas turbine based on these actual measured values; and a performance estimation module for evaluating performance of the gas turbine based on the deviation between the values of power output and exhaust temperature outputted from the performance computation module and the actual measured values of power output and exhaust temperature; wherein the gas turbine performance analysis system further comprises: an adjustment module for calculating a corrected value of the fuel flow rate based on the actual measured values of inlet air temperature, compressor pressure ratio and fuel flow rate, and inputting the corrected fuel flow rate value into the performance computation module; wherein the adjustment module is further for executing a computation to correct the fuel flow rate value based on the actual measured values of inlet air temperature, compressor pressure ratio and fuel flow rate according to a predetermined functional relation, replacing the measured value of fuel flow rate with the corrected fuel flow rate value, and inputting the corrected fuel flow rate value into the performance computation module, the performance computation module includes an adjustment function setting module for establishing the functional relation of the adjustment module so that a deviation between the values of power output and exhaust temperature, which were calculated by inputting a plurality of data sets including time-series data of a certain time period about the actual measured values of inlet air temperature, compressor pressure ratio, and fuel flow rate or data about a plurality of operating conditions, and the actual measured values is within a predetermined allowable range, and the adjustment function setting module comprises: a backward calculation module for revising fuel flow rate data of each data set so that the deviation between the computed values of power output and exhaust temperature, which are calculated and outputted by the performance computation module by using the data of each of a plurality of data sets, and the actual measured values is within predetermined upper and lower limits, a data retention means for storing fuel flow rate data revised by the backward calculation module along with the data about inlet air temperature, compressor pressure ratio, and fuel flow rate of the corresponding data set, and an adjustment function identification module for fitting the corrected fuel flow rate as a function of inlet air temperature, compressor pressure ratio, and fuel flow rate based on the corrected fuel flow rate, inlet air temperature, compressor pressure ratio, and fuel flow rate of a plurality of data sets stored in the data retention means.