A method for constructing a data structure of a proximate particle search is executed by a GPU, and computes a simulation of a physical object by converting three-dimensional digital image data to two-dimensional digital image data. The memory for a bucket is composed of buckets for dividing a memory space that is created in accordance with a space in which the physical object occurs. A writing step is provided in which a scattering function (GPU: vertex shader) is used by the GPU to read a particle coordinate and the particle number of a particle stored in the bucket and write the particle coordinate and the particle number to a two-dimensional image space.