First and second processing units execute the same control program to the same input data in parallel. An input/output unit generates the input data and receives one of two output data executed by the first and second processing units. A channel selection unit sends the input data to the first and second processing units and sends the one to the input/output unit by selecting the one from the two output data. In the first and second processing units, a control cycle synchronization unit generates a control cycle signal at a control cycle, a processor executes the control program, a data memory stores operation data including the input data, intermediate data being executed and output data executed by the processor. A diagnostics unit generates summary information by compressing the operation data and comparatively decides whether the summary information matches the other summary information of the other processing unit every control cycle.