To enhance the safety of a program that an arithmetic unit executes, by determining the connection relation of the connection terminals of the program storage device and an arithmetic processor to a data bus so that the arrangement of a plurality of bits constituting a program instruction is put back into the original one.

The odd-numbered data terminals of a microcomputer 1 are connected to the even-numbered data terminals of a program memory 3 so that a data terminal D0 of the microcomputer 1 is connected to a data terminal D1 of the program memory 3 and a data terminal D1 of the microcomputer 1 is connected to a data terminal D0 of the program memory 3. Further, the even-numbered terminals of the microcomputer 1 are connected to the odd- numbered data terminals of the program memory 3. Thus, they are so connected that the bit array of a program object stored in the program memory 3 is different from a bit array that the general computer 1 receives.