To provide an obstacle detector for a mobile object in which a mechanism such as an attitude stabilizer is unnecessary and a processing load can be reduced as compared to the Hough transformation.

A distance picture sensor 11 images slantwise and downward a special domain containing a plane on which a mobile main object 2 mounted on the mobile main object 2 is expected to travel. In a reference picture storage unit 15, the distance image of a plane within the scope of an angle predicted to be produced in the distance picture sensor 11 when the mobile main object 2 travels on the plane is stored in increments of predetermined width as a candidate reference picture. A difference picture production unit 12 produces a difference picture different from the distance picture produced in the distance picture sensor 11 from among a plurality of the candidate reference pictures stored in the reference picture storage unit 15. A reference distance picture selection unit 13 selects a candidate reference picture whose identical area becomes largest in the difference picture as a reference distance picture. A decision unit 14 defines an area other than the identical area of the difference picture as an area where an obstacle exists.