1. An analytical shell model forming apparatus for forming an analytical shell model for numerical analysis from a solid model to be analyzed, comprising: medial surface forming means for forming a plurality of medial surfaces for a same potion of the solid model; extracting means for extracting a difference portion by comparing the plurality of formed medial surfaces for a same surface of the solid model; medial surface candidate model forming means for forming a plurality of medial surface candidate models by using the extracted difference portion; and selecting means for selecting a medial surface model to be used for analysis from the plurality of formed medial surface candidate models.