This invention relates to an encryption communication apparatus using encryption processing. It is an object of this invention to obtain the encryption communication apparatus with excellent tamper-resistance while ensuring sufficient securities. A communicating unit 1 sends and receives communication data including a length field and a data field. A length comparing unit 6 compares a value of the length field and a minimum length value stored in the apparatus in advance. When it is judged that the value of the length field is less than the minimum length value, a controlling unit 7 controls an encryption processing unit 5 not to perform encryption processing or decryption processing of the data in the data field.