A new formula for the data analysis in the quasielastic light scattering with the sinusoidal electric field (QELS-SEF) was derived from the time-averaged autocorrelation function. This time-average process exactly corresponds to the actual experimental procedure which is performed with a correlator used in the QELS-SEF measurement. The new formula was applied to the power spectrum of the polystyrene latex solution in the heterodyne mode of QELS-SEF, and then was found to fit quite well to the experimental data. By this fitting procedure, we revealed the dependences of the electrophoretic mobility mu and the apparent diffusion constant D-app of the polystyrene latex on the frequency and amplitude of the applied ac electric field. Thus, the frequency dispersion or relaxation of D-app was observed in the same frequency region as the dielectric relaxation.