To accurately extract a sea-bottom surface even on condition that the sea-bottom surface slopes.

This sea-bottom area extraction device 10 is equipped with a range direction image extracting means 13 for extracting a plurality of two-dimensional images d for each of a plurality of range direction slope angles at an altitude c, a range direction slope angle outputting means 14 for outputting a range direction slope angle e corresponding to a two-dimensional image most looking like a sea-bottom image among the plurality of two-dimensional images d, an azimuth direction image extracting means 15 for extracting a plurality of two-dimensional images f for each of a plurality of azimuth direction slope angles at the altitude c, an azimuth direction slope angle outputting means 16 for outputting an azimuth direction slope angle g corresponding to a two-dimensional image most looking like a sea-bottom image among the plurality of two-dimensional images f, and a sea-bottom area outputting means 17 for outputting, as a sea-bottom area h, a plane having a range direction slope angle e and an azimuth direction slope angle g at the altitude c.

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