<p>Provided is an encryption device which can suppress increase of the size of encrypted data and prevent leak of plain data even if the accumulated data is analyzed. The encryption device encrypts data-to-be-encrypted smaller than the unit length of encryption. The encryption device stores management information indicating a used region already used for encryption among encryption regions decided according to the aforementioned encryption unit length. When encrypting new data-to-be-encrypted which is smaller than the encryption unit length, the used region among the encryption regions is confirmed according to the management information. Encrypted data is created by adding the new data-to-be-encrypted to an unused region other than the used region among the encryption regions. When the encrypted data is generated, the management information is updated so that the new encrypted data is contained in the used region.</p>
