PURPOSE:To eliminate the input and output of wrong data due to a ffult and to improve the reliability by checking the operations of an input and an output device right after a sequence controller is powered on. CONSTITUTION:When a power source is turned on to start processing, the internal memory of a microcomputer is initialized. Then, fixed check data (e) is outputted to the input circuit (h) through the output circuit (f) of an output device 12. The output data (e) is inputted from the device 10. Then, a check on whether the output data (e) is equal to input data or not is made and those processes are performed until the same data is obtained. When the data become equal, it is considered that the device 10 starts operating normally without any fault, so the next input processing is performed to fetch normal plant information from the device 10. The logical arithmetic of a sequence is performed on the basis of the information and the arithmetic result is outputted to a plant.