PURPOSE: To improve the reliability of the real-time multitask system where plural tasks are executed in time division by one CPU.

CONSTITUTION: When the use request of a module 10 occurs from a task 9, a module use service means 8 permits execution of this module 10 after referring to a module register table 1. Address information of modules 10 whose operations/non-operations don't directly cause a fault of the whole of the system are registered in the module register table 1, and the module which brings about a system error is deleted from the module register table 1, and therefore, the module 10 which brigs about a system error is not executed again hereafter.