A gateway apparatus comprising signal converter has service line concentrating unit and protection line concentrating unit, packet switch has a plurality of IP converters each line-concentrated to service line concentrating unit and protection line concentrating unit, first monitor monitoring responses from the plurality of IP converters through service line concentrating unit and protection line concentrating unit, second monitor monitoring responses from the plurality of IP converters through paths deferring from those of first monitor, failure determiner determining presence or absence of failures in at least any one of service line concentrating unit, protection line concentrating unit and the plurality of IP converters on basis of presences or absences of responses monitored by first and second monitors, respectively, and switcher switching protection line concentrating unit to be in effect to operate it as new service system when failure determiner has determined that any failure is present in service line concentrating unit.