A shortest path determining method includes: voxelizing a three-dimensional model to generate a voxel model; performing inversion processing on an area in three-dimensional space including the generated voxel model to invert an area set as voxels and an area not set as voxels; extracting an area including specific two points from the area set as voxels after the inversion processing, the area to be extracted allowing center of a specific sphere having a predetermined size to pass anywhere therein; determining a shortest path between the specific two points within the extracted area; and outputting the shortest path.