1. A vehicle surroundings monitoring device, comprising: a distance calculating unit which calculates, on a basis of a captured image by a single camera mounted on a vehicle, a distance between the vehicle and a physical body in real space corresponding to an image portion extracted from the captured image; and a physical body type judging unit which judges a type of the physical body in real space corresponding to the image portion; wherein the distance calculating unit determines a change in shape of the image portion or a change in shape of the physical body in real space corresponding to the image portion, in a predetermined period, determines whether to execute a first distance calculating process or a second distance calculating process based on whether the change in shape of the image portion or the change in shape of the physical body in real space corresponding to the image portion, in a predetermined period exceeds a predetermined level, executes the first distance calculating process, when the change in shape exceeds the predetermined level, of calculating a distance between the physical body and the vehicle, by applying the size of the image portion of the physical body extracted from the captured image to a correlative relationship between a distance from the vehicle in real space set on assumption of the type of the physical body and a size of the image portion in the captured image, and executes the second distance calculating process, when the change in shape is equal to or less than the predetermined level, of calculating the distance between the physical body and the vehicle, on the basis of the change in size of image portions of the physical body extracted from time-series captured images by the camera.