1. A travel distance detection device comprising: an image-capturing unit mounted on a rear part of a vehicle and configured to capture, at a predetermined downward view angle, an image of a predetermined region comprising at least one detection region; an alignment unit configured to align images of the predetermined region in a position of a bird's-eye view image, the images being captured at different times by the image-capturing unit; and a solid object detection unit configured to detect a solid object based on difference image data for the at least one detection region between data of the images captured at the different times and aligned by the alignment unit, wherein the solid object detection unit is configured to define at least one line in a direction in which a solid object is inclined when the image of the predetermined region is converted into the bird's-eye view image, count a number of pixels on the at least one line, each pixel expressing a certain difference with respect to the difference image data, locate an intersection between the at least one line and a reference line in a running direction of the vehicle, determine a position on an axis, the position corresponding to the counted number of pixels on the at least one line, produce a frequency distribution of the counted number of pixels along the reference line in the running direction of the vehicle, generate a difference waveform in the inclined direction in which the solid object is inclined due to conversion to the bird's-eye view image, and calculate a travel distance of the solid object based on time variation of the difference waveform.