A processor unit executes a failure detection program for a vehicle. The failure detection program includes a first failure detection process of a high priority level, a second failure detection process of a moderate priority level and a memory manipulation process of a low priority level. Each of the failure detection processes requests memory manipulation by generating an event as the need arises. When the memory manipulation process is activated, it performs the requested memory manipulation in the same order as the memory manipulation is requested so that execution of memory manipulation requested by one of the failure detection processes is not interrupted by execution of memory manipulation requested by the other of the failure detection processes. However, each of the failure detection processes itself can be executed interrupting the execution of memory manipulation process because of its higher priority level.