A technique to synthesize an accurate solid model from drawing data. Closed regions are detected based on top view data and a closed region list is generated. A tree structure is detected and the list is converted to symbol groups, each of which is a set of symbols representative of the elements of the contour line of the closed region and symbols representative of a connection relationship between the elements. When the converted symbol groups include a symbol group having a specific symbol, that symbol group is divided into a plurality of symbol groups based on that specific symbol. The symbol groups are matched, and the area of the closed region is detected. Symbol groups of closed regions of the same shape are classified and symbol groups of closed regions of the same area are classified. From the classified symbol groups, symbol groups of closed regions having a child of the closed region of the same shape and area are classified, and a solid model is constructed.