A device includes a storage unit that stores in a predetermined unit an identifier acquired from a packet transmitted from a transmitting device in association with a source address. The transmitting device receives from each terminal device connected communicatable via a network a packet including a source address, an identifier generated by adding a numerical value every time a packet is sent by the terminal device, and a session ID, and converts and transmits the source address. The transmitting device and the terminal device are connected to the device via a different network. The device also includes a determining unit that determines whether the identifier stored has monotonically increased; and a detector that detects a source indicated by a certain source address to be the transmitting device, if the determining unit determines that the identifier for the certain source address has not monotonically increased.