A compiler, that generates an object program file from a source program in which a plurality of procedures are written, compiles procedures, by regarding the procedures as source-program compile units, to generate corresponding object-program compile units. A plurality of object-program compile units generated are output to a memory together with the corresponding source-program compile units. When compiling a source program in which one procedure has been changed, the compiler compiles only the source-program compile unit corresponding to the changed procedure.