A method of producing an object program from an inputted source program with a compiler using a computer, includes the steps of: performing an interprocedural dataflow analysis of a variable associated with a procedure such as a subroutine, function, and initialization subprogram; analyzing and summarizing a procedure call effect into a variable region having a possibility of modifying the value of a variable upon a procedure call, a variable region having a possibility of using the value of a variable, and a variable region having a necessity of defining the value of a variable, in accordance with the interprocedural dataflow analysis; and representing data reference at the procedure call by using the summary information, in the dataflow analysis of a procedure part including the procedure call, performing an interprocedural dataflow analysis while discriminating between a case where there is a possibility of data reference and a case where there is a necessity of data reference, and performing an optimization process in accordance with the result of the last-mentioned interprocedural dataflow analysis.