1. A vehicle periphery monitoring device comprising: a plurality of imaging units mounted on a vehicle and configured to capture images of a vehicle periphery; an image brightness calculation unit configured to calculate a value corresponding to brightness of an image taken by any one of the plurality of imaging units, from the image taken by the one of the imaging units, a brightness correction unit configured to correct brightness of the images obtained by the plurality of imaging units on the basis of the value calculated by the image brightness calculation unit, an image transformation unit configured to perform a predetermined coordinate transformation process on the image with the brightness thereof corrected by the brightness correction unit, an image combining unit configured to combine the plurality of images after the coordinate transformation process into a single image, and an image display unit configured to simultaneously display the composite image obtained by the image combining unit and the image taken by the one of the imaging units.