A microstructure analysis method of a porous body using porous-body data in which positional information indicating position of a voxel obtained by three-dimensionally scanning the porous body is associated with voxel-type information indicating whether the voxel is a space voxel representing space or a matter voxel representing object, the method comprising the steps of: (a) a step of taking a curved surface solid formed by one virtual sphere or a combination of a plurality of virtual spheres as a virtual curved surface solid, and placing a plurality of said virtual curved surface solids so as to fill in the space voxels with curved surface solid voxels which are voxels occupied by the virtual curved surface solid, referring to the porous-body data, (b) a step of deriving information regarding a flow rate for each of the space voxels when a fluid passes through the porous body by executing a virtual-fluid analysis process based on the porous-body data; and (c) a step of analyzing microstructure of the porous body by deriving, based on information regarding the placed virtual curved surface solids, an equivalent diameter which is a diameter resulting when the virtual curved surface solid is converted to a sphere, and a volume of the virtual curved surface solid, for each of the virtual curved surface solids, deriving an average flow rate of the fluid passing through the virtual curved surface solid for each of the virtual curved surface solids based on both the information regarding the placed virtual curved surface solids and the information regarding the flow rate for each of the space voxels, and deriving a flow-rate-weighted mean diameter that is a weighted average obtained by weighting the equivalent diameter for each of the virtual curved surface solids with the volume and the average flow rate for each of the virtual curved surface solids. (a) a step of taking a curved surface solid formed by one virtual sphere or a combination of a plurality of virtual spheres as a virtual curved surface solid, and placing a plurality of said virtual curved surface solids so as to fill in the space voxels with curved surface solid voxels which are voxels occupied by the virtual curved surface solid, referring to the porous-body data, (b) a step of deriving information regarding a flow rate for each of the space voxels when a fluid passes through the porous body by executing a virtual-fluid analysis process based on the porous-body data; and (c) a step of analyzing microstructure of the porous body by deriving, based on information regarding the placed virtual curved surface solids, an equivalent diameter which is a diameter resulting when the virtual curved surface solid is converted to a sphere, and a volume of the virtual curved surface solid, for each of the virtual curved surface solids, deriving an average flow rate of the fluid passing through the virtual curved surface solid for each of the virtual curved surface solids based on both the information regarding the placed virtual curved surface solids and the information regarding the flow rate for each of the space voxels, and deriving a flow-rate-weighted mean diameter that is a weighted average obtained by weighting the equivalent diameter for each of the virtual curved surface solids with the volume and the average flow rate for each of the virtual curved surface solids.