PURPOSE: To eliminate the complexity of software and the decrease in performance by using a variable key code given in advance at write and using the key code at read to attain encoding/decoding directly through a data bus.

CONSTITUTION: A variable key code is stored from a main body device 7 in advance to a key storage section 3 via a file control section 5 and an encoding/ decoding control section 2. That is, when an I/O instruction transmitted from the device 7 is received by a file processing unit 1, a data is encoded according to the key code given in advance with an encoding section 4 inserted to an internal data bus and written in a file device 6. In case of read, the key code at write is stored in advance in the key storage section 3, an I/O instruction is transmitted and read is performed, the data in the device 6 is decoded by the decoding section 4 via a control section 5 and the result is transmitted to the device 7. The decrease in performance and the complexity of sofware are eliminated by using the variable key code to encode/decode directly a data on the data bus in such a way.