1. A vehicle surroundings monitoring apparatus comprising: a plurality of imaging units which capture an external region of a present-vehicle and output images; a display unit which is installed in an interior of the present-vehicle; an image combined unit which combines the images output from the imaging units and outputs a combined image; and a display control unit which displays the combined image output from the image combined unit on the display unit, wherein at least one of the imaging units is installed at a retractable side mirror of the present-vehicle, and the image combined unit extracts extracted images of an extraction range determined for respective images output from the imaging units, combines the extracted images to generate the combined image, and changes the extraction range and a boundary position between the extracted images constituting the combined image depending on extension and retraction of the side mirror.