1. Image processing apparatus, comprising: an image input unit configured to input a plurality of images in a time series, each input image including a boundary line extending toward a vanishing point on a plane; a reverse projection image generation unit configured to generate a reverse projection image of each input image by projecting information related to each input image onto the plane; and a boundary line detection unit configured to detect the boundary line from the reverse projection image by identifying a boundary line candidate on the reverse projection image.