A multicast message transmission device and a message receiving protocol device for guaranteeing a fair message delivery time for a multicast message. At the receiving protocol device, a release time of the received multicast message is set to a prescribed time specified to all receiving protocol devices of the same multicast group, and the received multicast message is stored until the release time, and then released to a corresponding upper level device. At the transmission device, each receiving protocol device is authenticated, and then the encrypted multicast messages are transmitted to the receiving protocol devices, while a prescribed decryption key corresponding to the encryption key of the encrypted multicast message is distributed the authenticated receiving protocol devices prior to transmissions of the multicast messages.