PURPOSE: To alter an incorporated microprogram from an outside by constituting a control storage of a ROM and a RAM and holding the copy of a program which should be stored in the control storage in an external storage.

CONSTITUTION: A micro instruction read out from the ROM 1 is stored in an instruction register 3 so as to be executed. An output from the RAM 2 is inputted in a decision circuit 4 and the data is written in the RAM 2 by a write circuit 5. If the read data of the address of the RAM 2 which is specified by a program counter 6 is '1', it is shown that the micro instruction stored in the ROM 1 of the corresponding address is effective. In such a case, the micro instruction read out from the ROM 1 is set in the register 3 to be executed. Meanwhile, if the read data is '0', the instruction read out from the outside through a data line 7 is carried out.