A DWS apparatus includes a coherent light source, a photodetector, a control unit which can measure an intensity autocorrelation function, a measuring unit which can measure a source-detector distance to obtain source-detector distance data, and a calibrating unit which adjusts the intensity autocorrelation function by using the source-detector distance data. The calibrating unit calibrates the intensity autocorrelation function by adjusting the time constant of the autocorrelation function based on a comparison of the source-detector distance to the time constant of the intensity autocorrelation function.
