1. A vehicle periphery monitoring device configured to recognize a monitoring object in the periphery of a self vehicle on the basis of a captured image by a vehicular camera, comprising: a symmetrical image portion extraction unit configured to extract from the captured image a first image portion and a second image portion which are line-symmetrical to each other in the horizontal direction; an expanded region setting unit configured to set a region which has a predefined size and contains the first image portion as a first expanded region; an expanded search range setting unit configured to set a region which is wider than the size of the first expanded region and contains the second image portion as an expanded search range; and an object class recognition unit configured to search in the expanded search range a second expanded region which has a correlation degree to the first expanded region or a mirror reflection image of the first expanded region not smaller than a predefined level, and recognize an image containing the first image portion and the second image portion as the image of another vehicle when the second expanded region is detected.