1. An image processing device for analyzing a surrounding image of a monitor-side vehicle obtained by an in-vehicle camera and detecting a presence of a vehicle around the monitor-side vehicle, the image processing device comprising: a region defining section, wherein there are provided a virtual background in a common image present at both a vehicle detection time and a predetermined time-period earlier time than the vehicle detection time, and a virtual vehicle moving in a predetermined speed in the common image present at both the times, and wherein a difference is obtained between a movement of the virtual background in the image from the predetermined time-period earlier time to the vehicle detection time and a movement of the virtual vehicle in the image from the predetermined time-period earlier time to the vehicle detection time, the region defining section defining for the image at the vehicle detection time a first region for which the difference is equal to or larger than a predetermined value, and a second region for which the difference is less than the predetermined value; a first image processing section which detects the presence of a vehicle from movement of a common feature portion present in the first region in the image obtained by the in-vehicle camera at the vehicle detection time and a corresponding region in the image at a time earlier than the vehicle detection time; a second image processing section which detects the presence of a vehicle according to whether a shape pattern analogous to any of a plurality of previously stored shape patterns exists in the second region in the image obtained by the in-vehicle camera at the vehicle detection time; and an output section which outputs, based on the detection results of the first image processing section and the second image processing section, a notification that the vehicle exists around the monitor-side vehicle.