A communication device comprising: a communication unit for transmitting and receiving signals with a reader/writer; a recording unit recorded with data and also recorded with a readout common key and a write common key as keys commonly used with the reader/writer, wherein the readout common key is different from the write common key; and a control unit for, when receiving via the communication unit from the reader/writer a cipher key update request signal including a readout common key identifier and a first cipher key encrypted with a key same as the readout common key recorded in the recording unit by the reader/writer, identifying, using the readout common key identifier received in the cipher key update request signal, the readout common key corresponding to the readout common key identifier, decrypting the encrypted first cipher key contained in the cipher key update request signal with the readout common key recorded in the recording unit, and recording the first cipher key obtained through decryption in the recording unit, when receiving via the communication unit from the reader/writer a readout request signal including a readout target address specifying a region of data readout source in the recording unit encrypted with the first cipher key by the reader/writer, decrypting the encrypted readout target address contained in the readout request signal with the first cipher key recorded in the recording unit, and transmitting the data recorded in the region specified by the readout target address obtained through decryption of the regions of the recording unit to the reader/writer via the communication unit, and when receiving via the communication unit from the reader/writer a cipher key generation request signal including a write common key identifier requesting for generation of a second cipher key, identifying, using the write common key identifier received in the cipher key generation request signal, the write common key corresponding to the write common key identifier, generating the second cipher key through a predetermined procedure, encrypting the generated second cipher key with the write common key recorded in the recording unit, and transmitting a cipher key generation response signal including the encrypted second cipher key to the reader/writer via the communication unit, and when receiving via the communication unit a write request signal including write target data encrypted by the reader/writer and write target address specifying a region of write destination of the encrypted write target data in the recording unit, decrypting the encrypted write target data and the encrypted write target address with the second cipher key recorded in the recording unit, and recording the write target data obtained through decryption in a region specified by the write target address obtained through decryption of the regions of the recording unit. a communication unit for transmitting and receiving signals with a reader/writer; a recording unit recorded with data and also recorded with a readout common key and a write common key as keys commonly used with the reader/writer, wherein the readout common key is different from the write common key; and a control unit for, when receiving via the communication unit from the reader/writer a cipher key update request signal including a readout common key identifier and a first cipher key encrypted with a key same as the readout common key recorded in the recording unit by the reader/writer, identifying, using the readout common key identifier received in the cipher key update request signal, the readout common key corresponding to the readout common key identifier, decrypting the encrypted first cipher key contained in the cipher key update request signal with the readout common key recorded in the recording unit, and recording the first cipher key obtained through decryption in the recording unit,