A method for preventing a return address from being falsified due to a buffer overflow during the program execution, and for detecting the buffer-overflow beforehand. When the return address is re-written during program execution, the debug function of the central processing unit is used to output an error. The falsification of the return address is detected through the error output. Then the falsified return address is re-written to a value stored in advance to enable the program to return to normal operation. When the falsification of the return address is detected, the executing program is terminated.