Program converting methods, apparatus and systems including a code analysis unit for performing lexical and syntactic analyses of a source code of an execution program, an optimization unit for transforming this execution program, and an output code generation unit for converting the transformed execution program into a machine language code. In addition, this optimization unit detects a calling procedure and a called procedure in a procedure call of this execution program analyzed by the code analysis unit, guards an evaluation of an argument described in the called procedure under a predetermined evaluation condition, and transforms the execution program so that the evaluation is performed when referring to this argument.