To allow for determining with high accuracy an event that represents an abnormal behavior such as sudden deceleration of a vehicle by a brake operation, and preventing an erroneous determination.SOLUTION: A speed signal generated in conjunction with the rotation of a wheel of a vehicle and a G value representing the magnitude of the acceleration applied to the vehicle are acquired. A speed change rate VA1 calculated from the speed signal is compared with a determination threshold TH1 to detect a rapid deceleration in S01. When sudden deceleration is detected at time ta, the G value VA2 in the range from time tb to ta is compared with the threshold value TH2, and when a condition of VA2≥TH2 is satisfied, sudden deceleration is confirmed in S02, causing an on-vehicle device to transmit the image data. Determination accuracy is improved by comparing the speed change rate VA1. It is confirmed that the G value VA2 is equal to or greater than the threshold value TH2 in order to prevent an erroneous determination caused by a wheel slip or the like.SELECTED DRAWING: Figure 2
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