Each of the host transmitting/receiving blocks of a plurality of I/O units connected to a CNC reads the destination specification data and a diagnostic result bit each time the host transmitting/receiving block receives a signal packet from the CNC. When the diagnostic result bit indicates the execution of a diagnosis, the signal packet contains diagnostic DO data, and the relevant DO signal output terminal is short-circuited to the relevant DI signal input terminal to form a diagnostic loop. The diagnostic packet signal passes through the diagnostic loop, the external signal transmitting/receiving block, and the host transmitting/receiving block and returns to the CNC through the host transmitting/receiving block of an adjacent I/O unit closer to the CNC. The CNC analyzes the packet signal to determine whether each of the I/O units is operating normally or not.