An encryption apparatus prevents plaintext data from leaking even if accumulated data is analyzed, while preventing the size of encrypted data from increasing. The encryption apparatus encrypts a data piece that is smaller than a unit length and stores management information indicating a used area within an encryption area defined based on the unit length. The used area is an area already used for encryption. When encrypting a new data piece that is smaller than the unit length, the encryption apparatus generates encrypted data by adding the new data piece to an unused area within the encryption area with reference to the management information. The unused area is an area not yet used for encryption. The encryption apparatus updates the management information to include an area for the new data piece into the used area, after generating the encrypted data.