PURPOSE: To prevent abnormal clock selection and the stop of a device function due to a fault, etc., of a clock selection control part as to the selection control system of a clock system having redundant constitution.
CONSTITUTION: The clock selection control system which controls the switching of a selector means 1 for respective parts in a device by a clock selection control means 2 according to clock break detection information of an input part is provided with a selector selection deciding means 3. When fault information on the clock selection control means 2 is generated, the selector control information of the clock selection control means 2 is disconnected and the switching state of the selector means 1 is held; when clock break detection signal on an in-operation system is generated, the switching state of the selector means 1 is inverted. Further, the state of the selector control information of the clock selection control means 2 is inverted according to the clock break detection information on the in-operation system even when the fault information of the clock selection control means 2 is not generated to invert the switching state of the selector means 1, and consequently the clock of a stand-by system is switched and outputted as an in-operation system clock.
