1. A method of generating a volume-panorama image, the method comprising: receiving pieces of image data of one or more volume images, the pieces representing an observation area of an inner portion of a target in a 3D manner; extracting one or more partial regions comprising one or more voxels from the volume images; determining a global conversion function representing a relationship of a conversion between a second volume image of the volume images and a first volume image of the volume images, the determined global conversion function being based on a conversion function representing a relationship of a conversion between one of the extracted partial regions of the second volume image and one of the extracted partial regions of the first volume image; and generating image data of a volume image to be combined with the first volume image from image data of the second volume image based on the determined global conversion function.