1. An image-based diagnosis assistance apparatus comprising: an image storage unit configured to store a morphology image representing the morphology of a specific organ including a tubular structure in the inside thereof and a function image representing a function level at each position in the organ; a structure region extraction unit configured to extract a structure region representing the structure dominating the organ from the morphology image; a partial region determination unit configured to determine a specific partial region in the extracted structure region; and a dominated region determination unit configured to determine a dominated region in the organ, and the dominated region being dominated by a structure in the determined partial region, wherein at least one of the step of determining the partial region by the partial region determination unit and the step of determining the dominated region by the dominated region determination unit is performed by using the function image.