In a chassis-type network relay device, fault tolerance for the continuity monitoring function of Ethernet (Registered Trademark) OAM is improved. A first management card and a second management card serving as a main management card and a backup management card have continuity monitoring units which generate monitoring frames for checking continuity between devices and transmit the monitoring frames to a given line card, respectively. Each of a plurality of line cards has a transmission source selecting unit which, when having received respective monitoring frames from both management cards, selects one of the monitoring frames and transmits the selected monitoring frame from a given port.