The present invention provides a code optimizing method for a program language processing system which can delete an unnecessary duplicate multiphase type definition and can avoid an unnecessary multiphase type from being instantiated. In this language processing system, all preprocessed source programs to be used as a source for generating an object program are scanned, a duplicate data type definition is deleted from the source programs with reference to a data type definition table, arranged for one object program, for registering a data type definition for data or a function in the source programs, instantiation of a data type definition which has been instantiated as needed is suppressed to optimize the source programs, and the optimized source programs are output in units of translation. Since a code size is reduced by the optimization, a code execution speed increases, and a compile time and a link time can be shortened.