1. A database encryption system, comprising: a database apparatus that comprises a database, and a database control unit that controls execution of database operation; and a user apparatus that is connected to the database control unit via a network, wherein the user apparatus further comprises: a key usage unit that manages key information for encryption and decryption; and an application response unit that receives a database operation command issued to the database apparatus, wherein the application response unit transmits to the database control unit, as data and/or metadata of the database operation command, encrypted data and/or encrypted metadata that has been encrypted according to key information of the key usage unit to cause the database control unit to execute the database operation, and the application response unit receives a database processing result transmitted by the database control unit, and returns a result of performing conversion or the decryption according to key information of the key usage unit, to an issuing source of the database operation command as a response to the database operation command.