Transition Edge Sensor (TES) is an energy dispersive X-ray detecting system with high energy resolution. The energy resolution of this system depends on the steepness of superconducting transition curve from normal to superconducting state, heat capacitance and the operating temperature. The TES is based on the dilution refrigerator cooled by about 100 mK. The energy resolution is calculated about 1-2 eV for the detector with maximum detecting energy as 10 eV. The energy resolution also depends on the superconducting current flowing through the TES device because the superconducting current is affected by the temperature stability of the refrigerator. The fluctuation of the superconducting current means the fluctuation of the X-ray spectrum peak center. We have developed the sensitivity correction system to stabilize the peak center of the X-ray spectrum. The peak center of X-ray spectrum correlates with heater power to keep the base temperature of TES device at a constant temperature. The peak center of X-ray spectrum is calibrated by monitoring the heater power at constant time interval using the correlation curve between the peak center of X-ray spectrum and heater power.