Conventional source-filter vocoders, such as WORLD, can quickly synthesize speech. However, the quality of synthetic speech is degraded due to speech parameters extraction errors. Therefore, this paper proposes minimum spectral error analysis, a speech analysis method that extracts speech parameters using Analysis-by-Synthesis (A-b-S), to improve the quality of speech synthesized by WORLD. We update speech parameters to minimize the error between the amplitude spectra of natural and synthetic speech. We developed the calculation process of the amplitude spectrum of synthetic speech from speech parameters to perform this analysis. A preliminary experiment shows that we have successfully constructed the calculation process.