To provide a computer system to detect generation of fault of a business program without requesting special hardware.

In this computer system, run-away of the business program is detected by constituting the system to write an initial value in a counter when a prescribed cycle is reached by using a program with priority lower than that of the business program, to increment (decrement) a value of the counter when the prescribed cycle is reached by using a program with priority higher than that of the business program and to execute an action processing for fault when the counter value reaches a prescribed value. Thus, since the generation of fault of the business program is detected without requesting the special hardware, various control systems are constructed by using a multi-purpose personal computer as it is.