An ultrasound observation apparatus combines ultrasound images generated based on an echo signal, the echo signal being obtained by converting an ultrasound echo into an electrical signal, the ultrasound echo being obtained by transmitting ultrasound to an observation target and by reflecting the ultrasound from the observation target. The apparatus includes: a memory that stores the ultrasound images in chronological order； a correlation unit that detects a moving amount and a moving direction as a three-dimensional correlation between an ultrasound image of a past frame and an ultrasound image of a latest frame； a position correction unit that moves a relative position of the ultrasound image of the past frame with respect to the ultrasound image of the latest frame based on the moving amount and moving direction； and a weighting unit that performs weighting processing on the ultrasound image of the past frame based on the moving amount.
