PURPOSE: To facilitate modification of a program by using program altering RAM incorporated in a one chip microcomputer, and a nonvolatile memory which is externally connected to the microcomputer, which stores program altering data and capable of serial communication.

CONSTITUTION: When alteration address data A1 which is set in an address data area 11a coincides with the output content of a program counter 1, an output permission signal is transmitted from a coincidence detection means 13a to a vector table 12a and only the data output of the vector table 12a is transmitted to a program counter value alteration means 14. When the output of the coincidence detection means 13a is simultaneously inputted to an interruption generation circuit 16 through an OR circuit 15, interruption occurs and a vector address 1 from the vector table 12a is inputted to the program counter alteration means 14 by the interruption. The means 14 refers to the content AAAA of the vector address 1 and alters the content of the program counter 1 to AAAA. Thus, the content of a ROM program can easily be altered.