1. An image processing device comprising: an image obtaining unit for obtaining a three-dimensional moving image showing a body part of a patient and an ultrasonic moving image showing the body part, the body part making a predetermined periodic motion; a characteristic part extracting unit for extracting, from a plurality of frame images forming the obtained three-dimensional moving image and ultrasonic moving image, a predetermined characteristic part having a shape that changes in response to the periodic motion; a phase obtaining unit for obtaining phases of the periodic motion captured in the three-dimensional moving image and the ultrasonic moving image, wherein, for at least one of the three-dimensional moving image and the ultrasonic moving image, the phases are obtained based on the shape of the extracted characteristic part; an associating unit for associating, for each phase, a position of the characteristic part in a frame image forming the three-dimensional moving image corresponding to the phase with a position of the characteristic part in a frame image forming the ultrasonic moving image corresponding to the phase based on the extracted characteristic part and the obtained phases; an image generating unit for generating a comparison image of each of the three-dimensional moving image and the ultrasonic moving image by aligning, for each phase, a position of the characteristic part in a frame image forming the three-dimensional moving image corresponding to the phase with a position of the characteristic part in a frame image forming the ultrasonic moving image corresponding to the phase based on the associated positions of the characteristic part and the phases; and a display controlling unit for displaying the generated comparison images in a manner allowing comparison therebetween on a display device.