This paper describes a microscopic modeling for urban surface layer, where a lot of various structures (i.e. building, bridge, pavement, and other facilities) exist. The authors found out a structural similarity between urban structure and porous media. Flow model and method of analysis in porous media were applied to the microscopic modeling for urban surface layer. First, 2-D numerical study was conducted on unsteady natural convection. Resultant model will be used for more precise 3-D computer simulation of urban warming and urban planning.