A new method for measuring the autocorrelation function in the quasielastic light scattering with the sinusoidal electric field specialized for the periodically stationary process proper to QELS-SEF. It has been found from the experimental results for a polystyrene latex solution with the new correlator that we can obtain the complex electrophoretic mobility mu* and the apparent diffusion constant D(app) in the frequency range much wider than the conventional QELS-SEF method.