The self-mixing interferometer is used to investigate the characteristics of resonant frequency of the microresonator, which is excited by a sinusoidally driven loud-speaker. The detected self-mixing signal is processed by the phase reconstruction method. The 1st-order resonant frequency of the microresonator is measured to be 4.437 kHz with full-width at half-maximum (FWHM) of 0.13 kHz. The measurement results are verified by the sinusoidal phase modulating (SPM) interferometer.