Data diode device with specific packet relay function, and method for specifying same

A data diode device (14) with a specific packet relay function is connected between an external network and an internal network. The data diode device (14) with a specific packet relay function has a first unit (21) and a second unit (22) connected by a signal line (Q). In the first unit, which is connected to the external network (15), a plurality of items of packet registration information are associated with a signal line (Q) and registered in a first reference table (T1). The second unit (22), which is connected to the internal network (16), has a second reference table (T2) that has the same content as the first reference table (T1). By activating the signal line corresponding to the packet registration information that includes the transmission origin IP address, destination IP address, and application data included in the packets received by the first unit (21), the content of the packets, and the fact that the packets transmitted to the internal network (16) were received, are transferred to the second unit (22). The second unit (22) generates packets based on the packet registration information and sends the packets to a second network. The data diode device (14) provided with a specific packet relay function relays the packets from an internal computer (12) to an external computer (11) via a one-way transmission path S for sending data in one direction.