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ATC (Air Traffic Control) communication between air traffic controllers and flight-crews is the most essential task in the ATC operations. This paper describes results from an analysis of the ATC communication in two (2) series of real-time (human-in-the-loop) simulation experiments. We analyzed communication-line occupancy rate in a modeled sector of Japanese en-route airspace. In the analysis, we focused on phraseology in the communication. We standardized air traffic controllers’ phraseology as well as flight-crews’ one, because individual difference of phraseology had an effect on time amount for the communication. Then, the line occupancy rate was computed on the standardized phraseology. Communication events were classified into some categories. Some of the items were regarded as replaceable with CPDLC (Controller-Pilot Data Link Communications) system. Comparing the occupancy rate between the case in which all the items were included and the case in which the replaceable items were excluded, the reduction effect of the CPDLC system introduction for voice communication line-occupancy was studied. From the analysis results, although fluctuation was identified, the CPDLC replaceable items proved to account for 24% in one simulation series, and 32% in the other series.