In order to reduce the load on the operator in monitoring a moving object in the moving picture signal supplied from the imaging device, the signal is displayed in one region of the display device. The moving route of the moving object which has entered the view of the imaging device is extracted, based on the signal. The first image data showing the image of a moving route of an extracted moving object is generated, and the second image data showing the image of the moving object is generated based on the signal. The overlapped image of the image of the moving route and the image of the moving object is displayed in another region of the display device together with the extraction time of the image of the moving object. The processing above is repeated to other moving objects which enter the view later on. The overlapped images for different moving images thus extracted are displayed adjacent to each other in the region.