A virtual vehicle which approaches to a monitor-side vehicle and a virtual background are defined in a camera image, and a region in which a virtual vehicle moves fast with respect to a virtual background is defined as a first region F1, and a region in which a virtual vehicle moves slowly with respect to a virtual background is defined as a second region F2. Then, the first region F1 and the second region F2 combined with an actual camera image. In the first region F1, in which the virtual vehicle moves fast, a monitored vehicle is detected by a movement aspect of feature portions in the region, and in the second region F2, in which the virtual vehicle moves slowly, a monitored vehicle is detected by pattern recognition.