A data transmission apparatus includes: a key exchange unit that exchanges an encryption key for encrypting and decrypting the data with an another apparatus; a first control unit that transmits information concerning the data transmission apparatus required in performing the key exchange by the another apparatus; a second control unit that transmits a first packet containing encrypted data and being set that the data size of the encrypted data is zero, to the another apparatus after the transmission of the information by the first control unit and until completion of the key exchange performed by the key exchange unit; and a third control unit that starts to transmit a second packet containing encrypted data provided by encrypting the data to be transmitted, to the another apparatus after completion of the key exchange performed by the key exchange unit.