To narrow down an abnormal portion in a plant controller to a scale of a circuit component.

The plant controller is composed of a processor 11 serving as a center of the plant controller, a memory 12 storing a control program, a programmable circuit 13 storing a random logic circuit, an I/O interface 14 outputting controlled plant data, and a self-diagnosis circuit 15 monitoring abnormal operation. The processor, memory, programmable circuit and I/O interface are respectively provided in pairs. A counter is provided for counting malfunction detection signals from the self-diagnosis circuit. When a malfunction is detected by the self-diagnosis circuit, which of the pair is to be used is sequentially determined in accordance with an output of the counter, rebooting is repeated, and the abnormal portion is determined by acquiring a counter value at a point of time where the malfunction disappears.