To provide a switching control system capable of secure fault detection of a control unit, and the control unit applied thereto.

A fault detecting circuit 20 outputs a fault detection signal TXON to another central processing unit according to a serial control signal TX and WD pulses, and the other central processing unit outputs the serial control signal TX to a communication line 4 for TX transmission in response to the fault detection signal TXON. Consequently, fault detection of the central processing unit can be performed by using the serial control signal TX in addition to the WD pulses. Therefore, even when fault detection can not be performed with the WD pulses although the serial control signal TX is not output normally, the fault detection signal TXON can be generated to securely perform the fault detection of the central processing unit.