1. A non-transitory computer-readable storage medium storing a program for causing a computer to execute a process, the process comprising: first generating first three-dimensional voxel data that represents a target object by using first tomographic images, in which a first region occupied by the target object is designated, among a plurality of tomographic images; extracting, from the first tomographic images, brightness values of voxels included in the first region; second generating a function for calculating a probability that a voxel is included in the first region by using the extracted brightness values; calculating, for each voxel among voxels in a voxel space that includes the plurality of tomographic images, a probability by using a brightness value of the voxel and the function; and third generating second three-dimensional voxel data that represents the target object by using the first three-dimensional voxel data and probabilities calculated for the voxels in the voxel space.