A technique that allows a dentist to accurately grasp movement of individual teeth tooth. A data processing device processes three-dimensional data of teeth. The data processing device includes an input unit to which the three-dimensional data including a bone of a part of a head and a plurality of teeth captured by an X-ray CT imaging device is input, a data processor that performs data processing based on the three-dimensional data input to the input unit, and an output unit that outputs the three-dimensional data processed by the data processor to an external device. The three-dimensional data further includes position information and is segmented into the bone of the part of the head and the plurality of teeth. The data processor compares the three-dimensional data captured at different timings based on a reference provided on at least the bone of the part of the head.