An ultrasound observation apparatus includes: a B-mode image generation unit； a region selection information input unit； a region-of-interest setting unit configured to set a region of interest of an ultrasound image to be subjected to processing suitable for an operation mode selected, in accordance with the input, from a plurality of operation modes for detecting a selected one of a plurality of pieces of characteristic biological information, by calculation using a region-of-interest setting method previously associated with the operation mode, based on a desired region selected in the region selection information； and a mode image composition unit configured to generate a mode composite image in which an operation mode image presented in two dimensions additional information obtained by performing processing suitable for the operation mode selected in accordance with the input on the region of interest is superimposed on a B-mode image generated by the B-mode image generation unit.
