In a ring type network, a master node sends a measurement packet to a slave node in the backward direction and the forward direction, receives the measurement packet which has been circulated among a plurality of slave nodes, stores a received time of the backward direction measurement packet and a sending time of the forward direction measurement packet according to time-keeping of an internal clock, sends a measurement result notifying packet showing a sending time and a received time of the measurement packet to the slave node; each slave node receives the backward direction measurement packet and the forward direction measurement packet, sends the received measurement packet to the next node, stores the received time of the backward direction measurement packet and the sending time of the forward direction measurement packet according to the time-keeping of the internal clock, receives a measurement result notifying packet, calculates a time correction value using the sending time and the received time of the measurement packet shown in the measurement result notifying packet and the sending time and the received time of the measurement packet stored, and corrects a time of the internal clock using the calculated time correction value.