A program to be executed by a computer is divided into a plurality of code blocks, and, a unique code block ID is allotted to each code block. At the moment when the execution of the program is started, the code block ID corresponding to the execution start address is written in a memory, and in the case when the control transits from the code block to other code block, by use of code block operation values obtained beforehand from these two code block IDs thereof, the code block ID in the memory is updated, and it is judged whether the updated code block ID in the memory and the code block ID allotted to the code block as the execution objective are identical or not so that a control flow error is detected.