The wireless transmission device connected to any one of subnetworks configuring an IP network including a server managing IP addresses, and connecting a mobile terminal to any one of subnetworks, includes an address storage unit storing identifying information specifying the mobile terminal as a sender of a message and the IP address assigned to the mobile terminal from the server, a notifying unit notifying other wireless transmission devices of the stored information, and a proxy response unit responding, as a proxy for the server, when handover occurs, a message containing the IP address, as an assigned IP address, already assigned to the mobile terminal that is the sender of the received message based on the information stored in the address storage unit.