A periphery monitoring device, comprising: a first storage that stores therein road surface information in a moving direction of a vehicle including a plurality of wheels, wherein the road surface information is a captured image of the road surface imaged by an imager provided in the vehicle, a controller that generates underfloor information that is an image of an underfloor region of the road surface below the vehicle, from the road surface information stored in the first storage, the underfloor region including at least part of each region of the wheels of the vehicle at the current position of the road surface imaged by the imager in the past; a second storage that is nonvolatile and stores therein the underfloor information generated at the position where the vehicle stops power supply of the vehicle; and a display that displays the underfloor information stored in the second storage when starting power supply of the vehicle, characterized by that, after starting power supply of the vehicle, the controller controls the display to display synthetic information, in which the underfloor information stored in the second storage and current ambient information, that is a captured image of current surroundings of the vehicle captured by the imager, are synthesized, and by that the controller controls the display to display the synthetic information comprising a combination of an image of the underfloor information in one image format and an image of the current ambient information in a color image format, the one image format being selected from an image format group of a monochromatic image, a gray image, and a sepia image. a first storage that stores therein road surface information in a moving direction of a vehicle including a plurality of wheels, wherein the road surface information is a captured image of the road surface imaged by an imager provided in the vehicle, a controller that generates underfloor information that is an image of an underfloor region of the road surface below the vehicle, from the road surface information stored in the first storage, the underfloor region including at least part of each region of the wheels of the vehicle at the current position of the road surface imaged by the imager in the past; a second storage that is nonvolatile and stores therein the underfloor information generated at the position where the vehicle stops power supply of the vehicle; and a display that displays the underfloor information stored in the second storage when starting power supply of the vehicle, characterized by that, after starting power supply of the vehicle, the controller controls the display to display synthetic information, in which the underfloor information stored in the second storage and current ambient information, that is a captured image of current surroundings of the vehicle captured by the imager, are synthesized, and by that the controller controls the display to display the synthetic information comprising a combination of an image of the underfloor information in one image format and an image of the current ambient information in a color image format, the one image format being selected from an image format group of a monochromatic image, a gray image, and a sepia image.