1. A plane detecting apparatus for detecting a plane from a captured image with capturing units, comprising: a pair of image capturing units located in a predetermined positional relationship, for capturing images of a mutually overlapping area containing a plane; a parameter storing unit for storing, as parameters, a rotation matrix and a translation vector for transforming one of coordinate systems of said pair of said image capturing units into another coordinate system; and a projection transformation matrix calculating unit for calculating a projection transformation matrix, said projection transformation matrix transforming one of said images captured by one image capturing unit to overlap said plane contained in said one image onto a plane contained in another of said images captured by another image capturing unit, by use of said rotation matrix and said translation vector; wherein said plane is detected using said projection transformation matrix.