PURPOSE: To output the source program into the mechanical program less in the number of instructions with efficiency, by deleting the specific instruction in the expansion form using the automatic return set, when the program of an intermediate word to be optimized satisfies the specified two conditions.

CONSTITUTION: The source program is changed into the program with an intermediate language, and when the program of the intermediate language optimized is converted into the objective program of machine word, variables VN indicating the automatic returned address, address GN indicating automatic returning point, and shunt area PSV of returned address are taken and it is discriminated that the following two conditions are satisfied. That is, the condition 1: VN←GN or VN←PSV are present on all the buses to automatic return, and the condition 2: the final execution of the buses in VN←GN. When these two conditions are satisfied, the instruction in the intermediate language or PSV←VN and VN←PSV in the expan- sion form during the automatic return set is delected to output the mechanical program less in the number of instructions.