The safe PLC1 and the safe slaves 2 are connected with each other
via the safe network 3. The safe PLC has the function of transmitting a
request of safety information by broadcast message to the safe slaves. The safe
slaves are each provided with a safety information transmission function for transmitting
safety information indicative of whether the safe slave is in a safe condition
or not; and a changing function for changing a priority of a transmission frame
which carries the safety information. As a result, when the safe slaves send back
safe responses at once in response to the broadcast message, safety information
(danger) with a higher priority is transmitted ahead of others to the safe PLC.