To understand the so‐called excess noise, we measured the complex impedance of a TES microcalorimeter, and derived the hernial sensitivity α, current sensitivity β, and heat capacity C. Making use of the obtained parameters, we found that the excess noise can be represented by two types of extra noise having similar spectra as Johnson and phonon noise. The contribution of the Johnson‐like, usually expressed by the M factor, depended on α M∽0.2α, which is consistent with the behavior of the excess noise studied in other groups. Pursuing the better energy resolution, we suppressed the dominant noise components by introducing a Si collimator to irradiate X‐rays only to the Au absorber, increasing the ADC resolution, and improving the temperature stability. So far, the energy resolution has been improved from 4.8 eV to 2.8 eV (FWHM).