A control device is provided that includes a power supply unit. First and second MOSFETs are serially connected to a sub power supply line in the power supply unit by connecting their respective drains to one another. Third and fourth MOSFETs are serially connected to a sub power supply in a control unit by connecting their respective drains to one another. By controlling these MOSFETs, power of the sub power supply is supplied to a load. The drain voltage of each MOSFET, and the voltage between the second MOSFET and the third MOSFET are monitored to determine a short-circuit fault and an open-circuit fault of the MOSFET, and a ground fault in the sub power supply line between the power supply unit and the control unit.