1. A fundus observation device comprising: a first image forming part configured to form a two-dimensional image of a surface of a fundus oculi of an eye based on optically obtained data; a second image forming part configured to form tomographic images of said fundus oculi based on data obtained by optically scanning a region of the surface of said fundus oculi corresponding to at least part of said two-dimensional image; an accumulated image generating part configured to generate an accumulated image by accumulating the formed tomographic images in a depth-wise direction; an extracting part configured to extract a first vascular territory corresponding to a fundus oculi vessel from said two-dimensional image formed by said first image forming part, and to also extract a second vascular territory corresponding to a fundus oculi vessel from said accumulated image generated by said accumulated image generating part; an alignment part configured to align said extracted first vasular territory with said extracted second vascular territory; and a specification part configured to specify a layer region or a boundary region between layer regions below the second vascular territory among regions of the tomographic images indicated by the first vascular territory, as a position of a vascular off region below a cross section of a fundus oculi vessel in the tomographic image when said extracted first vascular territory corresponds to said extracted second vascular territory as a result of the alignment.