A mobile communication system having a first communication system and a second communication system forwards forwarding data from the first communication system to the second communication system when switching from the first communication system to the second communication system is determined, the forwarding data being a data that the first communication system has already received from a core network device. A network device transmitting direct data to a mobile communication terminal via the second communication system is configured to count the number of switching times that the switching from the first communication system to the second communication system has been determined, for every user. The network device is configured to count the number of successful reception times that information identifying a user has been received via a forwarding path of the forwarding data, for every user. The network device is configured to judge whether the forwarding path of the forwarding data is normal or not on the basis of a ratio between the number of switching times and the number of successful reception times.