The object-oriented programming languages are widely used in the modern software engineering. The procedural type FEM codes are still used because it is very hard and cost consuming job to re-code them into object-oriented programs. In this study, a FEM program was newly designed and coded with C++, an object-oriented language. Also a special programming technique, auto-loading technique was developed for open structured program, with which the extension and maintenance can be carried out easily. For example, the additions of element libraries to an existence FEM program do not require any modifications of the program.