a nox purging system using a direct reduction type nox catalyst , wherein sulfur is purged while avoiding the occurrence of secondary sulfur poisoning , taking advantage of capability of recovery from deterioration caused by sulfur poisoning at exhaust gas temperature exhibited at normal operating zone , so that the influence of sulfur poisoning can be eliminated and nox is efficiently purged ; and a method of reactivating deteriorated catalyst therein . in particular , a nox purging system comprising exhaust gas passage and , arranged therein , direct reduction type nox catalyst , which nox purging system is fitted with first sulfur purge control means for performing such first sulfur purge operation that when exhaust gas temperature becomes higher than given set temperature during normal operation , not only is the oxygen concentration of exhaust gas decreased but also the exhaust temperature is raised to not below sulfur purge temperature .