Once the signal-tu-noise ratio (SNR) of the side- channel (SC) leakage trace is known, the intensity of the SC information leakage source inside the integrated circuit (IC) can be identified from measurements carried out outside the IC, SNR observation of SC leakage can also make it possible to set quantitative design targets to achieve the demanded leakage intensity. We discuss an improved method for identifying the SNR of SC leakage traces composed of multiple transient responses of IC switching current. The IC switching current repeatedly occurs as the IC runs the cryptographic operation since the cryptographic algorithm repeats a set of sub-operations. The method was applied to simulated and measured leakage traces to eliminate the effect of transient IC switching current caused before the target sub-operation was processed. As a result, a transient component more extensive than the signal component of side-channel analysis was identified in the decoupling capacitor configuration, where the convergence of the transient response is slow. In addition, the correlation coefficients obtained by the correlation power analysis, a major side-channel analysis method, were plotted as a function of SNR, and the plot of the simulated traces agreed with the theoretical curve. On the other hand, some errors remained in the plot of the measured traces.