1. A method for converting a program which reads in a source code of an execution program written in an object-oriented programming language and converts it into a machine language code, characterized by comprising steps of: reading out an execution program to be processed from a memory and examining whether a use of an object reference during a method invocation is an argument in which an element of the object is not used for a particular method invocation in the execution program; eliminating a predetermined code in the particular method invocation with dead code elimination, if the particular method invocation does not globalize an object reference during the particular method invocation and if a use of each object reference during the particular method invocation is an argument in which an element of the object is not used, for a predetermined method invocation in said execution program; and storing an execution program, of which a code has been eliminated, into a memory.