A simple method is described for evaluating the change in temporal coherence which occurs when a monochromatic light wave traverses a medium the properties of which fluctuate randomly. The degree of second-order coherence is formulated in terms of the amplitude and phase autocorrelation functions which are derived from the photocurrent produced in a light beating experiment. This method makes it possible to evaluate even a small deterioration in temporal coherence which is closely related to the frequency fluctuations in the light wave. The change in temporal coherence of a laser beam propagating through a slowly fluctuating gaseous path has been measured.