In a communication network system in which a master and a plurality of communication slaves are coupled through a high-potential side bus and a low-potential side bus in a daisy-chain manner, each of the communication slaves includes a control circuit, a resistance element, and a potential difference detecting portion. The control circuit controls communication with the master. The resistance element is inserted into the high-potential side bus at a portion located downstream of a point where the control circuit is coupled with the high-potential side bus. The potential difference detecting portion detects a potential difference between an upstream terminal of the resistance element and the low-potential side bus. The control circuit sets an ID value for communicating with the master in accordance with the potential difference detected by the potential difference detecting portion.