A power discontinuity occurring in any one of a plurality of I/O units connected to a numerical controller is detected. When the power discontinuity occurs in any one of the I/O units, the I/O unit detects the power discontinuity, and transmits the detection of the power discontinuity to an I/O unit in front of the I/O unit, at a timing different from a timing of an ordinary communication, in the form of a communication packet having a data pattern different from an ordinary data pattern. When the front I/O unit receives the communication packet, the communication packet is stored in a memory as power discontinuity detection information.