PURPOSE: To achieve system constitution with flexibility, by detecting that the operand section of an instruction word read out from a storage device is an even or odd number and designating the address mode.

CONSTITUTION: The instruction word system is used, which is set with the number of bits being a multiple of an even number for number of bits constituting the content in minumum unit for the address of a main storage device 5. The instruction word read out from the main storage device 5 is applied to an instruction word register 3, and the instruction is executed by decoding the operation section and the operand section. As a result of the operation section, if it is a jump instruction, a control circuit 2 detects if the operand is an even number, and if it is an even number, the content of the operand is applied to a program counter 1 and the jump instruction is executed with a normal address mode. On the other hand, if an odd number, the address designated at the operand is defined as n, and the content at the address (n-1) is read out and the jump instruction is executed with the indirect address mode.