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 also extract a second vascular territory corresponding to a fundus oculi vessel from said accumulated image generated by said accumulated image generating part; and a specification part configured to specify a position of a vascular cross sectional region corresponding to a cross section of a fundus oculi vessel in the tomographic image based on said extracted first vascular territory and said extracted second vascular territory.