Abstract: A microcomputer according to this invention which incorporates a bus switching circuit disposed between the ALU unit and memory unit, and the input/output unit, which carries out switching of the order of bit string and encryption of data and programs based on instructions from the ALU unit 2 or data or programs from the memory units 5 and 7. Encrypted data or programs are output to the outside of the microcomputer via the input/output unit 6, while the said encrypted data or programs that are input from the outside of the microcomputer are decoded and sent to the ALU unit 2 or the memory units 5 and 7.