PURPOSE: To improve efficiency of use of storage capacity in control storage by replacing a partial field of a microinstruction read out from control storage with altered information read out from control storage for expansion basing on the content of a specified field of a microinstruction register.

CONSTITUTION: A partial field of a microinstruction read out from control storage 5 is replaced with altered information read out from a control storage for expansion 11 basing on the content of a specified field of a microinstruction register and changed. Thereby, plural microprograms of which only a part of microinstruction is different can be unified, and consequently, number of steps of microprograms stored in the control storage 5 is reduced, and efficiency of use of the capacity in the control storage 5 is improved. As alteration is made by a microinstruction, the microinstruction can be changed to make the microinstruction most suitable for the operand of machine language.