A network system has lower switches and upper switches connected to the lower switches. Each of the upper switches includes a connection information notifying means for notifying connected lower switch list informations each of which includes an identification information on each of the lower switches connected to each of the upper switches, to the lower switches connected to each of the upper switches. Each of the lower switches includes a comparison determination means for comparing the connected lower switch list informations notified by the connection information notifying means, in order to determine a connection state between the upper switches and the lower switches.