The network relay device of the invention carries out data transmission to and from an opposite device to relay data in a network. The network relay device includes: multiple ports connecting with corresponding multiple ports of the opposite device via multiple links; and a link aggregation control module that aggregates the multiple links to establish a link aggregation, which is regarded as one logic link, and carries out transmission of a control frame signal including a synchronization bit, as well as data transmission, at each port via the link aggregation. In response to detection of occurrence of a trouble in at least one of the aggregated multiple links, the link aggregation control module causes a port that connects with a normal link without the trouble among the multiple ports to stop the data transmission and send the control frame signal including the synchronization bit set equal to a first value. Here the first value functions to stop the data transmission at a corresponding port of the opposite device connecting with the normal link.