Three-dimensional data acquisition means acquires three-dimensional data output from a three-dimensional ranging sensor for scanning a paved surface for aircraft using laser light. Extraction means extracts, from the three-dimensional data, real image data about a real image present on the paved surface and mirage image data about a mirage image corresponding to the real image. Foreign object detection means detects a foreign object left on the paved surface using the real image data and the mirage image data.