An I/O module includes a base plate, a plurality of universal circuits, and an option module. The base plate includes a plurality of connection terminals. A plurality of field devices is electrically connectable to the connection terminals. The universal circuits correspond to the connection terminals. The universal circuits are provided on the base plate, and configured to perform an input of analog signals from the field device, an output of analog signals to the field device, an input of discrete signals from the field device, and an output of discrete signals to the field device. The option module is detachably provided in the base plate. The option module is provided between a first connection terminal of the connection terminals and a first universal circuit of the universal circuits. The option module includes a first circuit configured to performing transmitting and receiving of signals between the first connection terminal and the first universal circuit. The first connection terminal corresponds to the first universal circuit.