PURPOSE: To detect whether or not a program runs away by judging whether or not an address which is executed at present is included in a memory area obtained from the module number of a module which is executed at present and a corresponding table at every prescribed time.

CONSTITUTION: In a step S1, the address during execution is read, and the module number during execution is read in a step S2. In a step S3, an execution start address and the size of the module are retrieved from the module size corresponding table. In a step S4, the address during execution is judged to be within a range or not as the retrieval result of size. When the address during execution is within the range as the result of the judgment, a system advances from the YES-side of the step 4 to a step 5 and the system is set to be normal. When the address during execution is not within the range in the step S4, the system advances from the NO-side of the step S4 to a step S6 and the program of a computer system is judged to be abnormal (during runaway).