A safety network system is constructed by connecting a safety PLC to a safety slave through a safety network. The safety slave obtains safety input informations from a plurality of safety devices connected thereto and transmits information based on the plurality of safety input informations in response to a request from the safety PLC. At this time, the safety slave determines whether or not a safety condition is satisfied based on the plurality of safety input informations, includes two notification modes, that is, a result notification mode in which a result of determination of safety determined by the above determination is transmitted and a detailed information mode in which the plurality of safety input informations are transmitted as they are as aggregated data, and returns a safety response in any of the modes according to a command from the safety PLC. In the result notification mode, the response can be returned at a high speed because an amount of data is small, and, in the detailed information mode, detailed information can be secured.