In a data diode device that includes a transmitting device, a one-way transmission path and a receiving device, in order to reduce packet loss that occurs when a receiving side is put in an overload state, the transmitting device includes a network load prediction function unit that predicts a network load of a receiving side network from the amount of communication of data sent from a transmitting side network, and a data control unit that discards IP packets of data sent from the transmitting side network when the network load predicted by the network load prediction function unit is determined to be higher than a predetermined value, so as to limit the amount of communication from the transmitting device.