Compared to radar, AIS has lower frequency and two stations(ships) not in line-of-sight may be able to communicate by diffracted wave. From the viewpoint of grasping the movement of another ship, which is the original purpose of AIS, non-line-of-sight propagation due to mountain diffraction is investigated in this study. Using the AIS data obtained at Japan Coast Guard Academy, the different states of the obstacles on propagation path, when it was able to receiveand when it was not possible, are investigated. The diffraction losses of some different conditions are calculated and compared. We aim to contribute to maritime traffic safety by estimating the receivable condition using the results.