1. A model simplification apparatus that, after input of a 3D geometric model to be subjected to numerical analysis to a computer, forms a surface mesh for numerical analysis on the geometric model while simplifying it, the model simplification apparatus comprising: a mesh formation section for forming a surface mesh by using a plurality of mesh elements on said input geometric model and registering said surface mesh as an initial mesh into a database; a geometry feature definition section for classifying said surface mesh into partial surface meshes having geometry features based on angles formed between respective adjacent mesh elements, and calculating feature quantities of said partial surface meshes classified; a feature shape input section capable of sequentially inputting each group of mesh elements of said geometry features as reference feature shapes; a similar feature shape search section for searching for groups of mesh elements of similar geometry features to said reference feature shapes, wherein the similar geometry features are comprised of groups of geometry features whose adjacency is equal to an adjacency of said reference feature shapes and in which the summation of differences of feature quantities between said similar feature shapes and said reference feature shapes is not greater than a threshold value; and a mesh simplification section for removing groups of mesh elements corresponding to said similar feature shapes from said initial mesh, and forming simplified mesh parts newly instead of removed similar feature shapes in spaces made by the removal.