Functional porous materials are widely used in a variety of industrial applications such as fuel cell electrodes. automobile catalysts, battery electrodes. and inorganic separation filters. Porous materials used in those applications typically possess irregular structure ranging from several tens nm to μm rather than a regular periodic structure in nm scale. Herein. we denote such porous materials as "irregular porous materials". In developing irregular porous materials with higher functionality, there are three major approaches: i.e., designing 1 ) materials, 2) interfaces, and 3) microstructures. Theoretical methods such as quantum mechanics have been widely used for the first and the second purposes. Toward the rational design of microstructure, which is more challenging than the rational materials and interfaces designs. numerical simulation studies have been widely performed. The key issue in those numerical approaches is the representation of the irregular porous structures in the simulation. To this issue, the traditional porous electrode theory. the percolation theory. or recently more sophisticated image-based approach is often used while novel approach is still waited to realize "design" beyond analyses. Toward the realization of the rational design of the complex microstructure, we have developed an original porous structure simulator and applied it to a variety of systems. We reviewed our recent applications of the simulator to solid ovide fuel cell, automobile catalyst, and dye-sensitized solar cell. Also our perspectives toward establishing a platform for the sciences and engineerings of irregular porous materials were described.