A rail detecting device includes: a starting-region rail-starting-point redesignating unit that shifts a starting region for detecting rails to set a modified starting region and designates modified starting points of two rails at the lower edge of the modified starting region when portions of the two rails are not detected based on rail segments in an image; a rail-candidate detecting unit that detects multiple lines in the modified starting region as multiple modified rail candidates, the lines being candidates of edges of the two rails; a starting-region-rail assigning unit that assigns modified rail candidates whose lowest ends are closest to the corresponding modified starting points out of the multiple modified rail candidates as tentative modified rail segments; and a starting-region-rail disappearance determining unit that establishes the tentative modified rail segments as modified rail segments when distances between the lowest ends of the tentative modified rail segments.