We have developed a new method of data processing for radio telescope observation data to measure time-dependent temporal coherence, and we named it cross-correlation spectrometry (XCS). XCS is an autocorrelation procedure that expands time lags over the integration time and is applied to data obtained from a single-dish observation. The temporal coherence property of received signals is enhanced by XCS. We tested the XCS technique using the data of strong H2O masers in W3 (H2O), W49N and W75N. We obtained the temporal coherent lengths of the maser emission to be 17.95 $pm$ 0.33 {mu}s, 26.89 $pm$ 0.49 {mu}s and 15.95 $pm$ 0.46 {mu}s for W3 (H2O), W49N and W75N, respectively. These results may indicate the existence of a coherent astrophysical maser.
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