PURPOSE: To prevent a ciphered program from being copied and in use in the ciphering program arithmetic unit mounted on an information terminal equipment or the like having a function decoding the ciphered program and executing it.

CONSTITUTION: An electronic computer 10 receives a ciphered part of a ciphered program loaded to an external memory 15, decodes the part according to a ciphering decoding program stored in advance in a built-in ROM 12 unable to be read to an external device and allows a built-in cache memory 13 to store the decoded program. Thus, an inhibit flag of a cache function is set to a control register 14 corresponding to a storage area of the decoded program stored in the built-in cache memory 13 to inhibit the decoded program from being read at an external bus and a CPU 11 executes a combination of the decoded program and the program being non-ciphered parts stored in the external memory 15.