A method of semiautomatic specification refinement and program generation using library modules, is described. Users write their specifications and modify and rearrange them so that they can be refined with the aid of the library modules. When a specification is given, a refinement system, called MAPS (module-aided program construction system) searches for library modules applicable to the given specification, replaces the specification with a more detailed description written in the operation part of the modules, and converts the refined specification into a program written in a programming language designated by the user. Case-like expressions or pseudo-natural language expressions are used for describing user's specifications and specifications for library modules.>