PURPOSE: To output an object program whose executing efficiency is satisfactory by obtaining insufficient information from a user at the time of optimizing and compiling a source program.

CONSTITUTION: When a compiler 1 is activated, a source program 6 is inputted and compiled. In an optimizing part 2, an optimizing advisability checking means 3 checks the advisability of the optimization of a variable in the source program 6, optimizing advisability information obtaining means 4 obtains the optimizing advisability information through an inputting and outputting device 8 from the user by an interaction when the advisability of the optimization of the variable can not be decided by the optimizing advisability information obtaining means 4, and optimizing means 5 optimizes the variable when the optimization of the variable is decided to be advisable, and when the optimizing advisability information indicates that the optimization of the variable is advisable. As the result, the compiler 1 outputs an object program 7 whose executing efficiency is satisfactory.