1. An image processing device comprising: a determination unit, included in a CPU-based control unit, configured to determine, based on a plurality of pieces of image data which is generated by continuously taking images of a specified area of field of view and input from an imaging unit, whether the area of field of view of the imaging unit has been changed; an image composition unit, included in the CPU-based control unit, configured to superimpose imaging regions of a plurality of images corresponding to the plurality of pieces of image data along a direction in which the area of field of view of the imaging unit has been changed, to generate composite image data when the determination unit determines that the area of field of view of the imaging unit has been changed; and a display controller configured to cause a display to display a composite image corresponding to the composite image data generated by the image composition unit, wherein the display controller is configured to eliminate an area of the composite image corresponding to older image data among the plurality of pieces of image data combined by the image composition unit, with elapse of time, and to cause the display to display the composite image.