An RTP packet generating unit 11 packetizes data into packets, and adds identification information to the header of each of the packets, the identification information identifying each of the packets. An RTP packet encrypting unit 13 divides data included in each of the generated packets into blocks, and encrypts the data included in each of the packets on a block-by-block basis using an encryption key which an encryption key sharing unit 12 shares with a receiving client in such a manner that, when encrypting a first block of the data, the packet encrypting unit encrypts it using the identification information for identifying each of the packets, which is contained, as an initial vector, in the header of each of the packets, and, when encrypting each subsequent block of the data, encrypts it according to an encryption method which uses an immediately-previously-encrypted block.