The present invention relates to a 3D ultrasound diagnostic forming a 3D ultrasound image only with volume data exiting within contour by automatically detecting the contour of a target object, comprising: a first unit for generating a region of interest (ROI) box on a 2D ultrasound image a second unit for detecting a contour of a target object in the ROI box and a third unit for forming a 3D ultrasound image by rendering volume data existing in the detected contour.
