There are disclosed embodiments for non-rigid image registration between 3-dimensional ultrasound and CT images by using intensity and gradient information of vessels and diaphragm. An ultrasound image forming unit transmits/receives ultrasound signals to/from a target object to thereby output electrical receive signals, and forms 3-dimensional ultrasound images based on the electrical receive signals. A CT image forming unit forms 3-dimensional CT images of the target object. A registration unit determines first and second objective functions associated with diaphragm and vessel regions of the target object, respectively, based on intensity and gradient information upon portions corresponding to the diaphragm and vessel regions in each of the 3-dimensional ultrasound and CT images. The registration unit performs non-rigid image registration between the 3-dimensional ultrasound images and the 3-dimensional CT images based on the first and second objective functions.