A data transmission method includes a step in which a first device generates a first encrypted packet by encrypting a packet addressed to a second device with an associated first encryption key. A device to be a transmission destination of the first encrypted packet is determined. A second encrypted packet is generated by encrypting the first encrypted packet with an associated second encryption key, and the second encrypted packet is transmitted to the determined device. The method includes determining another device and executing the transmission step if the decrypted first encrypted packet is not addressed to the device itself in the determination regarding whether or not the decrypted first encrypted packet is addressed to the device itself and of further decrypting the first encrypted packet if the decrypted first encrypted packet is addressed to the device itself.