To provide an automatic suspicious object detecting device which stably detects suspicious objects even if a residence time varies.

The automatic suspicious object detecting device includes: a photographing means 11 which photographs images; an object characteristics data extracting means 12 which extracts the movements of objects from the images photographed by the photographing means; a residence time computing means 13 which computes the times elapsed after object entry to the images from the movements of objects; a suspicious object determining means 14 which determines that the objects are suspicious ones in case that the residence times are larger than a threshold value; learning data 15 which store the residence times by the prescribed number of objects; and a data learning means 16 which finds the average value and standard deviation of the residence times and determines the threshold value from the average value and the standard deviation. By this, the device can stably detect a suspicious object even if the suspicious object is consecutively photographed.