PURPOSE: To use a one-chip microcomputer in which the content of a ROM before change is incorporated by changing a processing content by the processing content set by built-in memory and correcting the error of the processing content, etc.

CONSTITUTION: In the case that selective input 13 is selected so as to enable the use of memory 6 for correction, a central processing unit 1 issues an instruction to switch data input from the ROM 2 to the memory 6 for correction to a data selector 9 by a selection signal 44 in address designation for the ROM 2 of the central processing unit 1 when a high-order bit on that address coincides with data 8 to designate the high-order bit on the memory 6 for correction, and hereinafter, correction data 7 written on the memory 6 for correction from the outside is supplied to the central processing unit 1 before the operation of the one-chip microcomputer is started. An address signal 10 is supplied to the memory 6 for correction similarly as to the ROE 2, and it is constructed in the same structure as that of a ROM block, therefore, the readout of the ROM 2 can be performed similarly.