PURPOSE: To debug a program by substituting the instruction of an undefined external variable or function with another program at the time of coupling plural object programs.

CONSTITUTION: An object program coupling means 3 extracts each of external variables and external functions of object programs 1 and 2, and the means 3 substitutes the address part of the external variable or function with the address of the external variable or function when it is declared, but the means 3 uses the external variable or function as a parameter to substitute it with a program A call instruction and a program A 41 is coupled. The program A 41 is called when an undefined external variable or external function is referred at the time of executing an executive program 5. Thus, the program can be debugged though undefined external variable and external functions exist in plural object programs, and the easiness and the efficiency of debugging are improved.