1. A travel distance detection device comprising: an image-capturing unit configured to capture an image of a predetermined region; an alignment unit configured to align images of the predetermined region in position of bird's-eye view image, the images 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 between data of the images captured at the different times and aligned by the alignment unit, wherein the solid object detection unit counts the number of pixels which each express a certain difference on the difference image data and which are arranged in a direction in which the solid object is inclined when the image of the predetermined region is converted into the bird's-eye view image, then produces a frequency distribution of the counted pixels to generate a difference waveform, and calculates a travel distance of the solid object based on time variation of the difference waveform.