There is proposed a network system and the like achieving a packet relay transmission with high relay transmission efficiency according to Intermittent Periodic Transmit method (IPT). In order to achieve Intermittent Periodic Transmit in a mesh cluster, a communication control unit of a core node selects one of a plurality of communication units according to downlink priority and causes the selected communication unit to transmit a packet with a periodic interval. In order to transmit a packet from the core node to another slave node, a communication control unit of a slave node selects one of the communication units according to downlink priority, and in order to transmit a packet from a slave node to the core node, the communication control unit of the slave node selects one of the communication units according to uplink priority. Then, the communication control unit causes the selected communication unit to transmit the packet.