A network management device determines at least one of that an individual error occurs in a target terminal device when a difference between (i) a response time for the target terminal device and (ii) an average response time that is an average value of the response time for each of two or more terminal devices belonging to a group to which the target terminal device belongs is larger than a first predetermined value, and that a group error occurs in a target group when a difference between (i) a current average response time that is an average value of the response time and (ii) a previous average response time that is an average value of a previous response time for two or more terminal devices belonging to the target group is larger than a second predetermined value, the previous response time being immediately prior to the response time.