An ultrasound observation apparatus of the invention includes: an ultrasound probe portion capable of two-dimensional scanning of an ultrasound beam a transmission/reception control section for controlling a transmission/reception direction of the ultrasound beam by the ultrasound probe portion a B-mode image calculation section for generating a B-mode image on a scanning plane based on a result of scanning of the ultrasound beam a storage section for storing a predetermined sample image determined according to a shape of a treatment instrument for performing a treatment on a subject and a correlation calculation section for calculating a correlation value between the B-mode image and the sample image, wherein the transmission/reception control section moves the scanning plane such that the correlation value becomes the maximum.
