Plural of virtual curved surface solids, each of which is a curved surface solid formed by a combination of plural of virtual spheres, is placed so as to fill in space voxels, referring to porous-body data in which positional information is associated with voxel-type information (step S100). Information regarding a flow rate for each space voxel when a fluid passes through a porous body is derived by executing a fluid analysis based on the porous-body data (step S110). A flow-rate-weighted mean diameter Ru, which is a weighted average obtained by weighting an equivalent diameter R′i for each virtual curved surface solid with a volume Vi and an average flow rate Ui for each virtual curved surface solid, is derived based an information regarding the virtual curved surface solids and information regarding the flow rate for each space voxel (step S120).