PROBLEM TO BE SOLVED: To provide an image processing device and a medical image diagnostic device that enable a reference axis of an object included in a three-dimensional nuclear medicine image to be determined in conformity with a characteristic of a user practicing an imaging diagnosis.SOLUTION: The image processing device according to an embodiment comprises: an acquisition part a selection part a determination part an image generation part and a display control part. The acquisition part acquires a three-dimensional nuclear medicine image to be generated responding to radiation to be discharged by nuclide administered to a test analyte. The selection part holds information for each of users to determine a reference axis of an object included in the three-dimensional nuclear medicine image, and selects any of dictionary information responding to an input of a user from a plurality of kinds of dictionary information in which a characteristic of any user of a plurality of users is reflected. The determination part determines the reference axis, using the dictionary information selected by the selection part, and the image generation part generates a first cross section image indicative of a cross section satisfying a specific relation with the reference axis. The display control part performs a control to display the first cross section image on a display part.
